how can i keep the checkbox values after hitting the "previous" button on a hidden form?

I am new here and would like to ask you an urgent question, which i couldn't find an answer to:

I am creating a form where users can select products, enter their contact data and afterwards receive a confirmation.

My PHP-script consists of 3 pages. On the first page I have the contact data and the product selection, on the second i placed the code for an "overview", so the customer can check again, if his order is selected correctly and on the third and last page there you can find the confirmation.

My issue: on the second page i've made a hidden form, to keep the values from the first page (product, contact data). the products are selectable via checkbox. But how can I keep those checkbox-values, when the customer clicks on "preview" button (if he's not confident with his choice and if he wants to edit it)?

I would really appreciate an answer from you, many thanks in advance!