B6000/B7000 adhesive or superglue is the best to put the back on if it comes off. Unfortunately, the repair and the battery are more costly than the watch. Both glues can be removed if needed. The installation technique is to trial run for three times, watch the cable and the two wires (be very careful) and line up the sensors with the watch orientation, and glue once (like measure three times and cut once). Using a very light and thin coat of glue on the watch case not the back bezel and then hold together for about 10 minutes (placing upside down with a book or other weight…I just held it together) putting even wait on the back bezel and the watch. Ta - Done! It does not make sense that Apple charges some significant cash for their watch and can’t find a way to fasten the back bezel a little bit better, and fix it for a reasonable cost (why not just give anyone $199 off on a new one!!!). CoM-mm-Mon Apple.