First, make sure you’re looking at Gmail in a web browser. Next, click on the gear icon just below your avatar in the top right corner of the screen. Finally, select the “Go back to classic Gmail” button.
https://9to5google.com/2018/04/25/how-to-switch-classic-gmail-android-basics/
https://9to5google.com/2018/04/25/how-to-switch-classic-gmail-android-basics/