The safest way, albeit a little slow, would probably be to use audio editing software like Audacity, as long as you install the MP3 plugin for it, you can open the flac files and convert them to MP3.
For something faster, I use this program. It's a part of ad-supported freeware suite of all sorts of converters:
https://www.dvdvideosoft.com/products/dvd/Free-YouTube-to-MP3-Converter.htmYou can just drag and drop all the flac files, choose your output quality and it'll convert everything in one go, whereas with Audacity, you'd have to import each sound file separately, export each one separately. Audacity is mainly for editing, so it's not the most efficient as a simple converter.
If you go with the dvdvideosoft converter, just make sure that when you install it, to unclick all the other stuff it's going to prompt you to install. It usually asks you if you'd like to install some toolbars on your browser, change your default search engine, etc. You probably don't want any of that crap. Otherwise, the program itself is solid and I haven't had any issues with it.