how to replace front bumper on pontiac grand prix gt

If it's an aftermarket bumper, you're going to have a hard time getting it to fit properly. You may have to take it to a body shop. I had an aftermarket replacement bumper on my 2006 Grand Prix and the body shop had to apply heat to it and add a couple extra fasteners to get it to fit.

And, depending on what happened to the old one you may have other parts that are bent. Modern cars are meant to collapse fairly easily as a safety feature. The down side of that is it doesn't take much of an impact to knock things out of alignment.