Broken glass on the back of my iPhone

The glass on the back of iPhone is fused to the phone and cannot be repaired or replaced. The entire phone will have to replaced. The cost to you is dependent on whether you invested in AppleCare+ at the time of purchase. If you did, a replacement phone will cost $99 (in the US). If you didn't, you'd have to pay for an Out of Warranty replacement and for iPhone 12 Pro Max that cost is $599 (in the US).

See this link --> iPhone Repair - Official Apple Support

divyesh193 wrote:

I want to replace my xsmax back glass

That is NOT possible. The back glass is fused to the phone and the entire phone will have to be replaced. I provided a link two posts up with the prices to replace the phone.

I want to replace my xsmax back glass