can we use tempere glass for iphone 8 back side

There are no user serviceable components in an iPhone, nor does Apple sell

replacement parts. If you allow an unauthorized 3rd party to work on your iPhone

or do the work yourself, you void your warranty and are no longer eligible for a reduced price

Out of Warranty replacement.

If you are referring to a screen cover, such as the tempered glass screen protectors for the front screen, yes you can if you can find one.