iPhone
Open Settings → tap your name → Subscriptions.
Tap GymRats.
Pick the annual (or monthly) option.
The new plan starts at your next renewal date, and you keep your current plan until then.
Android
Open the paywall in GymRats and choose the annual plan, or
Cancel the monthly plan in Google Play (see How to cancel your subscription) and subscribe to annual once the month ends, so you don't pay twice.
Prices vary by region. Check the app for your price. For what's included, see GymRats Pro. Charged twice by mistake? See How to request a refund from Apple or Google.