If you've played Call of Duty: Mobile before, you'll automatically log in with the account you used last. If you'd like to log in with a different account, you'll need to log out first.
  1. Tap the Gear icon at the top of the main screen.
  2. Tap the Other tab in the far right end of the top row. You may need to swipe the tabs row to see Other.
  3. Tap the Logout button in the bottom right-hand corner.
Logging out will take you to the log in screen where you can log in using a different account.