To cancel your subscription, go to settings for your profile: The user icon in the far right of the channel's menu, then click on "My Settings" -> "Unlocked."
There you will see your active subscriptions, and you can then click "Unsubscribe" on the one you want to quit. Note that if you do not see "Unsubscribe", you will not be charged automatically, but your subscription will end when the active period is over. This will allow you to verify that no renewal will occur.
Address directly to that page: https://solidsport.com/user/profile/unlocked