From the Admin Dashboard, click on the Billing tab and then click Overview on the left hand side. Click the "Create charge" button in the top right corner.

Select all the students you'd like to switch billing frequencies for by clicking the checkmark to the left of their name. Once selected, click "Edit billing frequency" at the top of the table.

The Bulk update billing frequency window will appear. Select the correct frequency from the drop-down menu and click "Update".