The Kunena team has announce the arrival of Kunena 6.3.0 [K 6.3.0] in stable which is now available for download as a native Joomla extension for J! 4.4.x/5.0.x/5.1.x. This version addresses most of the issues that were discovered in K 6.2 and issues discovered during the last development stages of K 6.3
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You can backup your template but i advise you to use a template designed for kunena 1.5.11 else you will have some bugs and you can too backup too the language file.
I don't provide support by PM, because this can be useful for someone else.
The fact remains that you still have around 29 other unresolved errors in Kunena 1.5.9 that are just waiting to bite you when you least expect them to, until you upgrade to K 1.5.11.