Updating Sessions

Hi, I've got a multi page form and I'm using sessions to keep track of all the variables between the pages. SO I fill in page one of the form and add the variables to a session. Then I can go on completeing the rest etc. My problem is that I want my users to be able to go back to the pages they have already visited and edit the data (in the form) and then undate the variables in the session. But I can't update the session, so I thought if I called session_unregister and then session_register again then that would work, but it doesn't? How can I get around this? Thanks for any help, Jord