If Kunena is only used as a guestbook, there will probably also be writing rights for guests. If spam posts have been regularly deleted, these posts can be permanently deleted from the trash (Backend -> Kunena -> Trash -> Purge items). This will remove these posts from the database too.
However, if these posts have never been deleted, you will have to search the ‘Guestbook/Forum’ manually to delete them.

Afterwards you can import the Kunena tables into the new website database and then simply install Kunena 6.x over them. If there are also posts in the forum from registered users, you would also have to import the Joomla user tables into the new database. But I think you will do this anyway, as the Joomla users will remain the same in the new website.

Do you want a completely new forum just because of the difficulty with the migration, or do you want to fundamentally reorganise the forum?
If you want a new forum, the best way would have been if you had already removed Kunena 5.1 from the old Joomla installation via Kunena -> Tools -> Uninstall Kunena. Then search for Kunena via the extension manager to uninstall all Kunena extensions, if available. Then migrate Joomla to 4.4.3 and then reinstall Kunena.
Since you still have the old site, try it this way. But make sure you keep a backup of your old site.
This would have been the right way to migrate: docs.kunena.org/en/basics/upgrade/upgrade-k6-x
And this is a guide for a new installation for Kunena 6.x: docs.kunena.org/en/basics/installation/installation-k6-x

Which Kunena version was installed on Joomla 3? This was not mentioned in any post. I only see the possibility to remove Kunena completely and migrate again, provided that at least Kunena 5.1 or 5.2 was installed on Joomla 3!

1. you need all Kunena database tables from the Joomla 3 installation.
2. furthermore create a backup of the folder /media/kunena.
3. create a complete backup of your new website!
4. go to Kunena -> Tools -> Uninstall Kunena in the backend of the new site (it removes Kunena and the associated database tables)
5. import the Kunena tables from the old installation
6. install Kunena 6
7. if everything went well, import the avatars and attachments from /media/kunena

I don't know the Migrate Me tool. Which paths are entered in the configuration.php in the migrated Joomla 4 page?
 

Do you have a backup of the Kunena tables from the old Kunena 5 installation, or only from the migrated page?

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Before you do anything else, delete these menu items from your test page, then also from the trash (see image).
Then go to Kunena -> Tools -> Menu Manager and restore the menu.
After that, there should be a menu item for the forum in the main menu. Publish this menu item.
Choose the Kunena template Aurelia and check if the forum works.
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Yes, that is correct, because for Kunena 6.x the tables need to be adapted. It is likely that some posts will be added to your live site until the new test site is set up, so you should migrate these tables on the day of the move.
This can also be done on a second clean test site. Then remove the Kunena tables from the first finished test site and import the current tables.

But I just want to know if it is possible to change the width so that there will be a little "air" in the sides.

Can the field with the smileys be made bigger? It is a bit difficult to see which smileys they actually are

And is there an option to change the background colors etc.

There is no general solution for this. This requires custom CSS adjustments, depending on the Joomla template used.

Please read the linked article, then you will understand that neither you nor I can fix this. It has to be done by your provider/friend.
www.joomla-tips.org/joomla-troubleshoote...n-size-problems.html

Does it help if i got the Superuser acces ?

This is not possible. As I had already written, the necessary rights are missing. Even a superadmin doesn't have them if he can only access the website.Sorry as I am, you will have to wait until your friend is well enough to do it for you. You have until the beginning of August 2023, after which support for Joomla 3.x ends.
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What now ?

I have no idea what you did. But this limitation has nothing to do with Kunena and can also only be fixed by your friend/provider.

He told me to use a FTP to upload the file, but i really dont know how to.

For this, you need your own FTP access to the folder where the Joomla installation is located. But according to your statement, you only have admin access for your website. Therefore, it is not possible for you to upload anything via FTP.
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Rich, can you do anything about this from where you are ?

Unfortunately, I can't do anything from here, I wouldn't even be able to copy back a backup if something goes wrong, because I don't have the rights to do that. You can't give me rights that you don't have.
And trying to migrate without a backup is a risk you should not take.

So my question is if there is anyone here who can upload and install a 505 mb file for me.

Are you sure that this is an installation file? I can't imagine that a complete migration is done when installing this file. Maybe you should ask 9themestore how you can use this file.

Can you change the max from 256 to 600 mb, that will fix it

You can change that if you run a server yourself. Otherwise, you have to contact the provider and ask them to increase the limit.

This folder must be in the tmp folder of your Joomla installation. But if you only have admin access for the backend, you cannot upload the folder to the server. Are you sure that this package has to be installed if your website works now?

If it is an installation file, you can also install it via the tmp folder.
What happens when you unzip it? Is there one main folder afterwards, or are there several files or folders?
If it is only one folder, upload it to the tmp folder with your FTP software.
But if you see several folders or/and files after unpacking, then unpack the installation file into a folder you have created yourself and upload it to the tmp folder with your FTP software.
After upload go to "extensions -> Manage -> Install -> Install from Folder". Add at the end of address .../tmp/ your folder name and install your packet (see image).
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We are sorry if you want to leave Kunena, but it is your decision.
Go with your mouse to the User menu item at the top and select Create request from the submenu.