I have made some modifications to the classic theme that comes in wpforo, I know that when updating wpforo these changes will be lost.
It is possible to create a child theme in wpforo.
Can someone help me with a tutorial?
You should use the Update Safe way for changes. Just copy changes to current active theme /wpforo/ folder as it's described in this doc: https://wpforo.com/docs/root/forum-themes/theme-customization/
However, we don' recommend use old theme files, the latest 1.6.1 version comes with theme file changes, i recommend do the same customization on current version of template files.
@hector are you familiar with "diff" ? In my case, I have several modified templates, and it's much simpler for me to use something like MELD (Linux) or Notepad++ (Windows) to gain an overview of the changes made since the last update between versions. It removes the pain of having to review line-by-line.