@Console You can't do it yourself and I believe it was not possible a few weeks ago but I found this for you.
"cannot change your Stadia name directly in the Stadia app or on stadia.com. If you need to change it, contact us through Stadia Help. You can then speak or chat with a customer service representative who will help you change your Stadia name.
Note : To access Stadia Help from the Stadia app or from stadia.com, select your profile icon in the top right corner> Select settings> Go to Stadia name and avatar> Under "Stadia name" the link to contact us ."
Good luck and happy Christmas times.