Open the service and request cancellation. Two flavours:
- End of billing period - the service runs until the day you already paid for, then closes. The usual choice.
- Immediate - the service closes right away.
Cancelling stops future invoices for that service. It does not delete your invoices or your account, and other services are untouched.
Take your data first. A closed service's files and databases are removed - download what you need under Files and Databases before the end date, not after.