Where to add option for Next and Previous Page buttons using Multiform module?

Up front: I'm not a coder, but I must guide some who are...

We're setting up a 4 page form for a client using Multiform. The basics have been easier than expected - kudos to the creators.
But it doesn't have an option for direct navigation between pages. As is, on submit goes to a tween page that displays the links to the other form pages.

We want to change it so the submit is a "Next Page" action to the next page in the sequence and we want to add a "Back to Previous Page" button.
The question is, which Multiform file should we manipulate to add this feature: multiforms.module, multiforms_core.module, multiforms_ui.module?

Thanks in advance.
Cheers.