It is not possible to do so from the UI yet. If you don’t mind use the terminal, you can SSH into your Pi:
ssh -t umbrel@umbrel.local
(The password is your dashboard password)
Then run:
echo "$( jq '.name = "NEWNAME"' ~/umbrel/db/user.json )" > ~/umbrel/db/user.json
Don’t forget to change NEWNAME by your new username.
Be careful running this command. This file contains your hashed password and seed. Copy paste it and change only NEWNAME.
Example:
echo "$( jq '.name = "louneskmt"' ~/umbrel/db/user.json )" > ~/umbrel/db/user.json