Online shopping cart function

Hey guys, i have a part finished project that entails a step by step shopping cart (choose options as you go) which add to a total, then takes you to a paypal payment page. I need some alterations making on it, at the moment there is no way to step back in the process ie. go from option 3 to option 2, i need this adding in and along with a couple other bits and pieces. Paypal IPN is setup and ready along with other parts. I think the majority is finished, just some final pieces.