Converting HEIC files to JPEG

select the photos and export (file menu ==> export) as JPEG

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Have a look at this support document: Using HEIF or HEVC media on Apple devices - Apple Support

You can set your iPhone to take photos in a more compatible format or you can set it, so that the photos and videos will automatically converted, when you import photos from the iPhone.

Though capturing in HEIF and HEVC format is recommended, you can capture media using older formats that are more broadly compatible with other operating systems and devices:

1. In iOS 11, go to Settings > Camera.
2. Tap Formats.
3. Tap Most Compatible. This setting is available only on devices that can capture media in HEIF or HEVC format.
4. All new photos and videos will now use JPEG or H.264 format. To return to using the space-saving HEIF and HEVC formats, choose High Efficiency.

And to ensure the conversion to JPEG or H.264:

• When you share HEIF or HEVC media using other methods, such as AirDrop, Messages, or email, it's shared in a more compatible format, such as JPEG or H.264.
• When you import HEIF or HEVC media from an attached iOS device to Photos, Image Capture, or a PC, the media might be converted to JPEG or H.264, if you go to Settings > Photos (on your iPhone). In the "Transfer to Mac or PC" section, deselect "Keep Originals" to have the media being converted when importing.

Which version of MacOS X are you using? If you Mac has been updated to High Sierra, Preview and Photos can convert HEIF images to JPEGs when exporting photos.

On an older system you can convert HEIF images to JPEGs at this website: heictojpg.com