When you create a Mixer account you may be auto assigned a username or you may want to make some changes due to branding or Gamertag updates.
At this time Mixer does not automatically update your Username to match your Xbox Live Gamertag.
There are a couple ways to update your username (max characters are 20):
You can change your username via your account page at https://mixer.com/me/account/security using the "Change Username" panel.
via Mobile App
You can also change your username via the Mixer mobile apps, available on Android and iOS. Log in via your Microsoft account and tap the "Profile" icon on the bottom right, then tap the pencil icon on the right hand side of your current username.
How many characters can a username have?
Usernames can have anywhere from 1 to 20 characters. Letters, numbers, and underscores only. Spaces are not supported
I keep getting a message prompting me to link a Microsoft account
In order to make changes to your username, you'll need to have your Mixer account linked to a Microsoft account, you can learn more on how to do this here.
After linking your Microsoft account, you may need to sign out and then back in to update your username.
I updated my username but it didn't change
Some times your changes may be cached. Give it a few minutes, refresh and it should show your new name.
If you're on the mobile app, you may need to relaunch the app in order to see the changes on your end.
Note that you can only do this once every 30 days and the site will automatically redirect your old channel to your new one during that time.