In the flow action(Action) you have the option to customize the action button labels or hide the buttons.
If you want to have a previous button to go back to the previous stage you will have to implement change stage on click of button.
If the requirement is only to distribute a big form into multiple screens and go back and forth, then you can check the concept of screen-flows where you will have the option to go to previous and next page page.
1.Open the FlowAction in which you have configured your respective section and navigate to Action tab and select the checkbox "Hide the default action section buttons" . This will hide the default button.Refer Screenshot.
2. Refer the section "pyCaseActionAreaButtons" in work- and customise this section in your class and use it to customise button .Note: This section is referred in "pycaseactionarea" rule.