The solution by CW_Kevo is probably the best one but here are some other possible options that can help depending on the situation:
It's unusual to mess up the files in
vendor unless manually editing them (which you shouldn't do), but if that happened, you can just delete the
vendor folder and run
composer install to re-install all the files like they were.
To copy a fresh version of all Flarum and extension files in
public, you can run the
php flarum assets:publish command. This also happens automatically after the Flarum installation and whenever you enable an extension.