My new dryer has a 4 prong cord and I have a 3 prong outlet. I have purchased the 3 prong cord - it is a Whirlpool Model LER7648PQ0

How do I change out the cord.

If you have 4 wire service to t he outlet, it would be better to change the outlet. The old 3 wire system uses one one wire for both neutral and ground. You have various colors, maybe 2 blacks, black and red, and black with a red stripe. Whatever, they connect to one of each. Then the white, green, bare whatever all connect together.