Open the service and press Request Cancellation. You have two options:
Cancelling stops further invoices for that service. It does not delete your client account, past invoices or other services.
Take your data first. Once hosting is switched off its files, databases and email are deleted. Make a backup in DirectAdmin and download it to your computer before the end date. The same goes for a VPS - copy your data off it before the service ends.
To cancel a domain, turn off its auto renewal. If you are moving the domain elsewhere, transfer it before it expires.