How do I change an old 3 prong dryer cord to a new 4 prong cord. My dryer has 3 on it but new house has 4 prong receptale.

Get yourself a 30 amp 4 wire male "pigtail" to match the outlet. Look for a NEMA number on the face.

Remove the old cord, usually held in by a two screw cable clamp, or should be, from the dryer.

You will see a terminal strip with three lug studs usually nutted,sometimes clamping screws. You should see the center one has the white wires going into the unit. Here is where you should see a jumper strap or wire connecting the white wires to the metal frame of the unit.

Remove this and discard.

Center lug is white, the other two get the black and red, don't matter which one, and the green from the cord needs to connect to the metal frame, may need some sort of lug held by a tight sheet metal ,better if machine thread, screw.

Tighten the cord clamp. Done.

Then where does the dryers ground wire(green, yellow stripes) go