1. Log in to Control Panel
2. Click the Sites tab
3. Click the Manage link for the appropriate site
4. In the Site Info section, click the Change Billing Period icon
5. In the drop-down menu, select the billing period you wish to switch to
6. Click the Change button

For information on how you will be billed for changing your billing period, see:

How will I be billed for changing from monthly to annual billing?

or

How will I be billed for changing from annual to monthly billing?