Manage Your Google Account.
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To set up a default account on Android, follow these steps:
Open Settings on your Android device.
Scroll down and tap on Google.
Tap Manage Your Google Account.
Swipe to the Data & Privacy tab or similar (varies by Android version).
To change the default account:
Open the Gmail app or any Google app.
Tap your profile picture (top-right corner).
Select Manage Accounts on this Device or similar.
Remove other accounts (optional) or sign out of your current one.
Add your desired default account back and ensure it's the first account logged in.