Converting HEIC to JPG is one of the most common file conversion tasks, and fortunately one of the easiest. Here are three methods, ranked by convenience.
The fastest option is a browser-based converter like fwip. Drop your HEIC file onto the page, get a JPG back instantly. The conversion happens entirely in your browser — the file never leaves your device. This works on any operating system and handles batch conversion too.
On Mac, you already have built-in conversion. Open the HEIC file in Preview, then go to File → Export and choose JPEG from the format dropdown. You can also select multiple HEIC files in Finder, right-click, and use Quick Actions → Convert Image. Both methods are free and built into macOS.
On iPhone, you can avoid the problem entirely. Go to Settings → Camera → Formats and select "Most Compatible." This makes your camera save photos as JPG instead of HEIC going forward. For existing photos, share them via email or AirDrop — iOS automatically converts to JPG when sharing to non-Apple destinations.
For bulk conversion of hundreds of files, IrfanView (Windows, free) or XnConvert (cross-platform, free) can batch-process entire folders in seconds.