Open the program Adobe Photoshop; import the double chin photograph.
Select the Filter drop down menu and select 'Liquify' from the menu.
Use the Forward Warp Tool (default tool in the liquify filter) to push the bottom part of the double chin upwards. Make sure to zoom in and make the Forward Warp Tool as big or as small as needed for the chin.
Click and drag up below the double chin line, drag down above the double chin line until the double chin is gone. Make sure there are no extra markings on the face before you save the photo.