If you're cancelling one individual contact's membership you're in the right place.
Alternatively, if you're looking to cancel several memberships check out the article Cancelling Multiple Memberships.
To cancel a membership:
Go to Contacts.
Search for the Contact.
Select the Contact to edit.
Under that Contact's Memberships, select the relevant membership you wish to cancel.
Select the Cancel button to change that membership to Cancelled.
That's it! The membership will now be cancelled for that Contact.
If you need to re-activate the Membership you can simply click Activate and their membership will be re-activated.