Which file to change “About me” title in?

There are proper ways of doing things, like overrides of styles, custom filters/snippets, translating, and there are old good “change original sources” as well.

But every one of them starts with you downloading sources (or copy your wp-content\plugins\anspress-question-answer\ folder to some easy to use local place) and searching through them (I do it using a smart text editor, ctrl+shift+F usually searches in all files in a project).

You will find matches for the string you want to change. Then you decide, do you have enough IT expertise to generate a snippet for it, like this one, do an override if possible, or you will be fine with just editing the sources.

Keep in mind, that editing sources have a drawback: you will have to never update the plugin, if it’s updated, all your changes are gone. There are special tools, “version control systems”, that developers use to easily merge different sets of changes, but that is usually not easy to use.