Changing your subscription length takes effect at your next renewal date, not immediately. For example, if your monthly subscription renews on the 5th and you switch to an annual plan, the change takes effect on the 5th, and that's when you're billed the annual amount.
Follow the instructions below based on how you originally subscribed.
Google Play
From the Strava app on Android:
Go to the You tab and select the Settings icon in the top right.
Tap Your Strava Subscription > Manage Your Subscription > Change Plan.
Select your new subscription length.
Apple App Store
From the Strava app on iPhone:
Go to the You tab and select the Settings icon in the top right.
Tap Your Strava Subscription > Manage Your Subscription > Change Plan.
Select your new subscription length.
Strava Website
Log in to your account and navigate to Settings in the menu under your profile picture in the top right.
Select My Account in the left-hand menu, then select Change Plan.
Select your new subscription length.