HEIC is a new photo format introduced since iOS 11. Compared to old image formats, it needs less storage space. However it brings us some complications at the same time. For example, you will find you can’t open or view HEIC files in Windows, Android, even Macs that don’t run the latest MacOS.
To prevent HEIC compatibility problems, Apple allows us to change the Camera option to take and save photos in JPG format from Settings > Camera > Formats. Change the camera capture format from High Efficiency to Most Compatible. Then your camera will capture and save photos in JPEG format. Note that this option will change the video format from HEVC to H.264 at the same time.
Other than that you also have the option to convert HEIC to JPEG when you export photos to PC or Mac computers. Just head to Settings > Photos, scroll all the way down to the bottom, then change from Keep Originals to Automatic.
After that, you can connect your iPhone to PC via USB, then copy and save photos from iPhone to PC in JPG format. In this way, you can automatically convert HEIC photos to JPG upon transfer. The above setting is to recognize and convert only those HEIC photos upon transfer. It will not alter PNG or other files or formats.
If you didn’t customize the photo format setting in the first place, use a third party photo editor to convert HEIC to JPG or JPEG on iPhone.