Even with this guide and the comments, I managed to destroy my watch following it. The removal of the back, there is a thin film over the back plastic. I heated up the back enough for the film to peel away from the plastic back, but the film half still stuck to the watch. There were also the problem with the locking pins, which on further inspection may be able to be removed through the watch strap area. Remove the watch strap first!!!!!
When trying to prise the watch from the metal casing (nb this was before I read the comments and saw about the locking pins), the watch suddenly came free... Unfortunately because of this, the ribbon cable for the touchscreen tore along with it. What is happening is as you are prising the back off, you are essentially breaking the locking pins, which are plastic and suddenly give way.
THIS GUIDE NEEDS UPDATING..... AS IT STANDS, IT IS NOT SUFFICIENT TO CHANGE THESE WATCH BATTERIES WITHOUT POTENTIALLY DESTROYING THE WATCH. I would recommend reading a few other guides. RIP! ;(