You can change the look of your Avatar by going to the Avatar tab and selecting the one that appeals to you. There are several free Avatars and many for sale for Robux.
You can further customize your Xbox avatar via the Avatar Editor. To access it:
- Log into your Roblox account via the Xbox app
- Navigate to the Avatar tab
- Press the Y button
On this screen, you can edit various aspects of your avatar's look, such as Clothing, Body, Animation and Outfits.