On your billing page:
1. Select the plan you would like.
2. If needed, fill in your billing details.
3. Check the box next to 'I understand that this subscription will automatically renew'.
4. Click "Subscribe".
If you would like to cancel a paid or trial subscription you can select the 'Free' plan, check the box next to 'I understand that this subscription will automatically renew' and then select 'Subscribe'. This immediately sets your subscription to 'Free' and you will no longer be billed, but will still be able to access your account.
Note: The 'Subscribe' button is treated as 'Save' or 'Update'.
In case of a cancelled paid subscription, you can use the paid features until the end of your billing cycle.
If you want to completely delete your account and all documents related to it, have a look here.