sizonov_stas Instead of importing, you can as well connect to your old database after a fresh install of beta-5. This requires 2 steps :
1) In config.php set the name of the old database
2) Run <your flarum utl>/admin (like this, admin without the extension) and then enter the password of your database so that it can be converted to the new format.
Before this it is good to make a dump (backup) of your database