Sign out of all Google accounts on your browser or device
Sign in to the account you want to set as default first
Sign in to any additional Google accounts after that
On desktop, open a Google service like Gmail or Drive to confirm the first signed-in account is the default
On mobile, set the preferred account as the primary account in your device settings if needed
If the wrong account stays default, clear browser cookies and sign in again in the desired order
In Chrome, use separate profiles if you want different default Google accounts
On shared devices, remove unwanted accounts before signing in to the preferred one first