Drupal7 multistep module - how to skip validation on previous
I'm not familiar with the multistep module, but in Drupal 7 this functionality can normally be accomplished by using #limit_validation_errors. It would probably be best to alter the form(s) that you need with hook_form_alter (or a variation of it) in a custom module. A basic untested example of what I described:

function MYMODULE_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  if ($form_id == 'THE_RELEVANT_FORM_ID') {
    $form['THE_PREVIOUS_BUTTON_KEY_NAME']['#limit_validation_errors'] = array();
  }
}