I want to update my IOS to version 15.7.2.1

To update to the most recent, compatible version of iOS, please see: Update your iPhone, iPad, or iPod touch - Apple Support

And: If your iPhone, iPad, or iPod touch won't update - Apple Support

If the above Apple articles do not help, please post back with relevant information. 

To undated my phone

You cannot update to iOS 15.7.2.1, but you can update to iOS 15.7.7 if you have an iPhone 6s, iPhons 6s Plus, or iPhone 7.

iPhone 8 and higher can go to iOS 16.5.1

Update your iPhone or iPadAppleOfficial Apple Support › en-us

My phone battery died quickly