G'day lecascadeur78.
That's a good question!

There is no
Kunena configuration setting that applies to
all users. Each user has their own
Prefered Message Ordering (sic) setting [First post first | Last post first]. Some users prefer to make the change individually. I think
Kunena is designed well in this regard.
You can change the individual setting for yourself (as the system administrator) or for other users by using the
User Administration feature at the backend. I'm sorry, I haven't written detailed notes about this feature at the
Wiki
.
For users to change their settings themselves, things get a bit more complicated. It's an unfortunate side-effect of choosing a
Profile other than "Kunena" - see
Integration »
Profile Settings - that you
also make it difficult for your users to change their forum preferences. This is
one reason why I choose
Profile = Kunena. It's not the only one.
I assume you've chosen
Profile = JomSocial. There is another way for them to make this change: have your users go to
<your-domain>/forum?func=myprofile&do=show, select
Look and Layout to make the change themselves.