You can easily access the Meta Avatar by heading to Settings, then Profile.
As a quick note:
Meta Avatars are only available on Quest 2 and Pro devices. If you're using FitXR on an older Quest device, or on a Pico headset, then you won't be able to access this feature.
Accessing the editor
When you navigate to Settings, and then click Profile over on the left hand side, you'll be met with this screen if you've not already set up a Meta Avatar.
All you have to do is click the "Create Meta Avatar" button on the lower left side. That will then direct you out of the FitXR app and into the Meta Avatar Editor. When you're done, it'll bring you straight back into the FitXR app and update your avatar for you.
Then how do I edit it?
Exactly the same process - except that same button will now say "Edit Meta Avatar". You'll be taken out to the Meta Avatar Editor again, and brought back into FitXR when you're done editing.