Start by clicking the Account Settings tab in the drop down list under your name in the top right corner.

2)    Next click on the Plan & Billing info tab in the left margin or click the blue box on the right named View Plan & Billing Info

3)    To change your payment plan, click the Change Preference box under Payment Preference.

4)    Click the circle next to the payment plan of your choosing.  Your selection should have the blue circle next to it.  After you make your decision, click the Save Payment Settings button at the bottom.