It’s no really hard to do it, just be careful the worst that happens to me is the heart rate monitor isn’t working and i don’t care maybe because i did open the watch so many times.
to open the rear cover first remove the battery is easy and the heart rate monitor ribbon cable attached under the cover with your nail or finger try to do it moving it back and forward you will find out it’s not glued or nothing after that remove the plastic like square frame just lifted off and be careful with the heart rate cables you will see the antennas of the band . To remove the band one side of the screws is removable and the other it’s not so take the screws out and with something pushed inside out to remove the other part you will find out I will posting some pictures. Be careful and good luck mine is working fine but no the heart rate monitor.
Thanks to Zeblaze for facilitating all the parts