Depending on the service you used to purchase the Premium subscription (website, Android, or iPhone) there will be slightly different ways to downgrade your account to free and cancel any active and renewing subscriptions:
Website Premium subscription purchases:
If you're currently a Strava Premium Member on a monthly or annual billing plan and would like to stop the automatic renewal that occurs at the end of the billing cycle then you'll need to downgrade your account. This is done from Your Account Details page. If you're logged in click here. Click "Manage" under Membership Status, then just click the link under "Other Options" to Downgrade your account.
Note that this won't end your premium membership instantly and you'll remain a Premium Member with access to all the premium benefits until your membership expires on the Expiration Date as indicated on Your Account Details page.
If you purchased your Premium subscription using the Strava App for Android, your payment and any recurring payments are handled by Google Play. To check on the status of your Google Play subscription, or to cancel any 'Active' recurring subscriptions, navigate to the Strava App in the Google Play store. For more information on managing Google Play subscriptions, see this Google Support page.
iPhone Premium subscription purchases (iTunes):
Similar to above, Premium purchased using the Strava App for iPhone will be processed by the iTunes store. For more information on managing iTunes subscriptions, see this Apple Support page.