Cjcrux
These are commands you type in your terminal window once you have established an SSH connection with your server. The first command:
composer remove <package>
... instructs composer to remove the package that you want to see deleted.
The name of the extension has to be exactly the same as it was during installation, e.g. composer remove flagrow/upload. Just replace require with remove.
The second command:
php flarum cache:clear
... instructs php to clear Flarum's cache.
The third command:
rm -f assets/rev-manifest.json
... removes the file assets/rev-manifest.json. That's also a kind of cache clearing as this file will be generated anew whenever it's missing.