Hello [name]! Thanks for joining the GMC. Before making any posts in the Tech Support forum, can we suggest you read the forum rules? These are simple guidelines that we ask you to follow so that you can get the best help possible for your issue.
In post-2.3 GMS, rooms are reordered under Asset Browser > Quick Access > Room Order.
If you are just starting out, DO NOT just look up random tutorials on YouTube. Many of them are for older versions and have instructions that no longer apply. Start with the ones on this page that are current as of the time of writing.