hrvoje_hr no, Flarum is not designed in a way that makes downgrading possible. If there was any migrations executed during the update, there's no way to rollback only those migrations. Most Flarum minor updates and some of the patch updates have such migrations. In theory migrations could be implemented in a way that allows rolling back step by step like in Laravel but Flarum has not implemented this feature.
Only solution is to revert to backup, or manually inspect each database change and revert it by hand.
If there was no migration on the other hand (during a patch release or some extension updates) you could simply require an older version of a package with Composer and clear Flarum's cache.
Under some specific situations it would be possible to run an older version of Flarum connected to a newer database version, but that's completely unsupported and likely to lead to errors immediately and/or when finally updating later depending on the kind of database changes.