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 Roblox website. Log into www.roblox.com with the credentials of your Xbox Roblox account. You will be able to select hats, hair, arms, legs, and torsos which will change the appearance of your Roblox Avatar on Xbox.