Tried to update from Flatboard codename-LES AÇORES-build 180225 to flatboard 2.5 and everything seemed to go well but when I tried to login to my backend it kept telling me invalid password.

I re-installed my backup and the password worked fine and everything was back to normal.

Did I miss something as I followed the update guide on https://flatboard.org/view.php/plugin/page/p/docs.

Hi nootkkan,
The version you used is very old, and version 2.5 uses the new username encryption with a key file generated when installing Flatboard.
In your case, you simply had to restore the data folder either manually via a backup made just before the update, or by the backup plugin.
Anyway, I'm glad you got your forum back with all of its data.
Have a good day.