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 under "Unlocked subscriptions", and you can then click "Cancel subscription" on the one you want to quit. Note that if you do not see "Cancel subscription", you will not be charged automatically, and 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