How do I change my username
You can't, you have to contact support and ask them to do it for you.
https://support.google.com/stadia/answer/9622629?hl=en-GB
As per FAQ:
You cannot change your Stadia Name directly in the Stadia app or on stadia.com. If you need to change your Stadia Name, contact us on the Stadia Help Centre. From there, you can choose to call or chat with an agent who can help you change your Stadia Name.
You can't, you have to contact support and ask them to do it for you.
https://support.google.com/stadia/answer/9622629?hl=en-GB
As per FAQ:
You cannot change your Stadia Name directly in the Stadia app or on stadia.com. If you need to change your Stadia Name, contact us on the Stadia Help Centre. From there, you can choose to call or chat with an agent who can help you change your Stadia Name.