ondrejj what Flarum version are you running ?
FontAwesome was updated between beta 7 and beta 8, but the icons are almost identical between v4 and v5 of FontAwesome. Do you have maybe a screenshot of what the icons look like on your forum so we can get a better idea of what you mean by different ? Have you overridden the default icons on your forum maybe ?
To update Flarum from beta 7 have a look at the beta 8 announcement discussion, it contains the update instructions. If you're already on beta 8.1, there's nothing to update to.
