If the iDevice Manager is not installed yet, download the program from here.
Start the iDevice Manager and click the button "HEIC > JPG Converter" or press in menu "HEIC > JPG".
Use the mouse to drag the High Efficiency iPhone Image files from the computer into the empty area of the iDevice Manager (Drag 'n Drop).
Alternatively, the files can be picked from a folder on your PC by clicking on the "Select Files" menu entry.
To start converting, click the button "Start converting"
Choose the destination folder, if you want to convert multiple files or the file name for one file.
Depending on the file size, runs the converting faster or slower.