I'm going to do my best to walk you through this, LOL.
Go to Acct Management Page and log in.
On this page at the bottom click Update Renewal
Choose how often you want to pay and click continue
Next Page, Top Right click Manage Accts
On this next page is where you really need to look... Look on the left side for SMALL RED letters that say "edit".
That will bring up a page for you to edit how and what you pay with.
Good Luck
(I have to give Queen Mum a HUGE thanks here because she had to walk me through this several times over ICQ!)