On the payment page, please uncheck "Use the address on my tax return as my billing address". You should then be able to change the address for the credit card information.
If that doesn't work. Please clear your browser cache (or use a different browser) and try again. This FAQ has instructions on how to clear the cache in several popular browsers: How do I clear my cache (browsing history)?
If you are still not able to resolve this issue, please contact our phone support team at 1-888-829-8608 for more help.