If you click on ‘My account’ in the menu, you will see a page with your subscriptions on it. There you will see a blue button with ‘Cancel subscription’.
If you click on this, the subscription will be cancelled and no further payments will be taken from your account. You will still be able to use +Test Zone or +Class View until your subscription ends, which will be when your current month's subscription period has ended.
All of your work will be saved in 'My Writing' and all your tasks in your personal workbooks, but you will not be able to access the +Test Zone or +Class View features. You can still use all the free features of Write & Improve.