@sgtjimmyrustles You will still need to install from a fresh image (It would be possible to update from Jessie to Stretch manually, but it's quicker to backup roms/configsm write the image and restore imho)

You can use this image https://retropie.org.uk/forum/topic/16307/retropie-4-3-15-stretch-images-for-testing/ - it will become the next RetroPie image (with a few bugfixes if there are any issues we missed)