To update your account's payment method:

1) Log in to your account

2) Click on "My Account"

My Account

3) On the Left Side-bar, click on Plan & Billing

4) Click on Change Payment Method

Change payment method

5) Update your billing information and click Update.

Update billing information 

If you are not seeing this option on your account, you are most likely not logged in as the administrator, but as a sub-user. Try to log into the dashboard using your administrative account information and you should see and make changes in the Plan & Billing area.