I’m having iPhone 6s Plus but the updates are not getting installed. Please do help

You should be able to update the 6s Plus to iOS 15, but are you sure that your phone is the 6s Plus? Go to settings>General>About>Model Name and check to be sure. If you have an iPhone 6, you can’t update to iOS 15.

What version of iOS is it running now?

If it is running 12.5.5 or shows 12.5.5 as the available update, you do not have a 6s plus. You have a 6 plus. 12.5.5 is the end of the line.