Subscriptions can be cancelled anytime in the GiD shop.
Log in
Go to My account > My orders
There you will see a list of your subscriptions. Click on View on the one you want to cancel.
Click on Cancel
This will cancel any renewal for this subscription, but you are always welcome to subscribe back.
The current licence will expire when the licence period is over so, if you have a GiD monthly subscription starting every 1st day of the month, and you cancel it on the 15th, you will be able to use GiD for the rest of the month.