How do I get the back off s6 to reset phone

[AC Question]

Phone has turned itself off and cannot restart

 

[belodion]

You'll have to try a simulated battery pull, since the back is not removable by the user.

 

[smvim]

Phone has turned itself off and cannot restart

If your S6 is completely unresponsive (and you're pretty sure the battery has at least some charge left in it) try holding down the Power button for at least ten seconds to be sure it's powered off, then try to restart it.
If you've been having battery issues recently and suspect it needs replacing, ifixit has a nice step-by-step guide on pulling the battery in a S6, it's not a trivial process and not something to try unless you have all the proper tools. Samsung used a lot of glues and adhesives instead of physical connectors for this phone so be prepared (be patient taking things apart, make sure you have required adhesives on hand to put things back together):
https://www.ifixit.com/Guide/Samsung+Galaxy+S6+Battery+Replacement/44648