Open the app's Settings screen, tap "Premium service", then tap "Premium plan" and subscribe to the monthly or annual plan. This hides all ads. Premium also unlocks all theme colors and lets you set the wallpaper and date background colors as often as you like. You can cancel your plan at any time.
If you previously purchased the one-time "Remove ads" option, no additional payment is required. Even after switching devices or deleting the app, as long as you use the same Google account you purchased with, ads will be hidden automatically when you open the app. If a "Reactivate ad removal" button is shown on the Settings screen, tap it. (If you already have a Premium subscription, open "Premium service" on the Settings screen and use "Restore" instead.)