Open Billing in your account and choose Manage billing, then cancel the plan. Your site stays online until the end of the period you have already paid for, and you can renew the plan in Manage billing until then.
When the plan ends, the site is taken offline and permanently deleted 30 days later. To keep it, open the site under Websites and choose Reactivate before then: the site comes back exactly as it was, with its files, database and address. Buying a new plan for the same domain does not bring the old site back.