Sign out of all Google accounts
Sign in to the account you want to use first
Sign in to any additional Google accounts after that
Open Gmail, Google Drive, or another Google service to confirm the first signed-in account is the default
On mobile, remove all Google accounts from the app or device
Add the desired default account first
Add the other accounts afterward
If needed, set the preferred account as the primary account on your device
Clear browser cookies if the old default account still appears
Reopen Google and sign in again in the correct order