Editing page output HTML

Hey, thanks for the plugin, very helpful.

I would like to know how I can slightly edit the code which generates the output HTML of each page created by the plugin. I want to have HTML which should be added for each page (without editing all of my posts ofc).

Example: Add a button before the "Next" button to skip 2 pages forward. (Not really a required feature, just to explain my meaning).

I am a developer so I think I can handle the changes myself but I am new to WP so I don't know where and how to find the right place to edit the plugin code.

Thanks again !