Ok, I entered my folder where I have Flarum installed and I used the commands posted in the official release thread by Toby, but I get this:
[u@host ~/public_html/wpank/labs]# sed -i -e 's/"minimum-stability": "beta"/"minimum-stability": "beta",\'$'\n'' "prefer-stable": true/' composer.json
[u@host ~/public_html/wpank/labs]# remove=""; require=""; for extension in akismet approval auth-facebook auth-github auth-twitter bbcode emoji english flags likes lock markdown mentions pusher sticky subscriptions suspend tags; do remove="$remove flarum/$extension"; require="$require flarum/flarum-ext-$extension=^0.1.0"; done
[u@host ~/public_html/wpank/labs]# composer remove flarum/composer-installer $remove -q
PHP Warning: PHP Startup: Unable to load dynamic library '/usr/local/lib/php/extensions/no-debug-non-zts-20121212/xdebug.so' - /usr/local/lib/php/extensions/no-debug-non-zts-20121212/xdebug.so: cannot open shared object file: No such file or directory in Unknown on line 0
[u@host ~/public_html/wpank/labs]# composer require $require
PHP Warning: PHP Startup: Unable to load dynamic library '/usr/local/lib/php/extensions/no-debug-non-zts-20121212/xdebug.so' - /usr/local/lib/php/extensions/no-debug-non-zts-20121212/xdebug.so: cannot open shared object file: No such file or directory in Unknown on line 0
[u@host ~/public_html/wpank/labs]# composer update
PHP Warning: PHP Startup: Unable to load dynamic library '/usr/local/lib/php/extensions/no-debug-non-zts-20121212/xdebug.so' - /usr/local/lib/php/extensions/no-debug-non-zts-20121212/xdebug.so: cannot open shared object file: No such file or directory in Unknown on line 0
[u@host ~/public_html/wpank/labs]# php flarum migrate
PHP Warning: PHP Startup: Unable to load dynamic library '/usr/local/lib/php/extensions/no-debug-non-zts-20121212/xdebug.so' - /usr/local/lib/php/extensions/no-debug-non-zts-20121212/xdebug.so: cannot open shared object file: No such file or directory in Unknown on line 0
Migrating Flarum...
Nothing to migrate.
DONE.
[u@host ~/public_html/wpank/labs]# rm -Rf assets/*.js assets/*.css storage/cache/* storage/formatter/* storage/views/*
[u@host ~/public_html/wpank/labs]#
Umh, and yeah, it seems to have updated anything (in the admin panel I still have Beta3).