Here's how to do it:
Login to your Pixelfy.me account and click on your name on the top-right corner of the screen
Next, from the drop-down menu, click the option that reads "Billing History"
This page will show you all your billing information where you can:
Check and download your Pixelfy invoices
Update your invoice address
Change your Credit Card if you've registered one
Remove your Credit Card if you've registered one

Note: If you are using PayPal, the option to add or remove a Credit Card will not be shown