You can change Roblox display name from ‘Account Info’ settings. Your display name needs to satisfy 4 different criteria before updating. Roblox display name and username are different things.
Roblox game guide, tips, tricks and answers for frequently asked questions.
You can find Roblox user ID number from your profile URL. Roblox User ID can only be accessible from PC or Mobile browser URL. Roblox player ID used to quickly find your player profile in game.
No, You can not play Roblox offline. There are many reasons for that which are given in this post.