Open the app's Settings screen, tap "Premium plan", and subscribe to the "Premium service" (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, simply tap "Reactivate ad removal" and ads will be hidden again for free. (If you already have a Premium subscription, use "Restore Subscription" instead.)