I need to convert some photos to JPEG from HIEC format

If your pictures are in Mac Photos, then you can select them and use File>Export xx Photos -- this brings up a dialog in which you can choose the format of the exported image.

If your pictures are in a folder in Finder, you can open them in Preview and Export, choosing the format you need.

However, if you open the photo using the Preview app, you will have to click on the Export link and it will then convert the photo to a JPEG format.