Open the menu in the app by tapping the three bars in the top right corner. Tap your email address to open your account. By tapping your phone number, you should see the option to "Remove Number" at the bottom of the screen.
Adding a phone number can only be done by booking a bike or car. The checklist will show your phone number still needs to be verified. Tap "Verify mobile number" at the bottom of the screen, follow the steps in the app to add your phone number (back) to your account.