There are basically two styles of lens designs when it comes to the aperture ring, which is what you are replacing. Some lenses have screw heads exposed on the face of the lens mount, shown below.
On lenses of this type the mount itself has a lip which holds the aperture ring on. Usually the hardest part on this type of lens is getting the screws out. Frequently Nikon has glued them in. It sometimes helps to heat them briefly with a soldering iron to loosen the glue. Once the screws are removed, lift the mount off. The aperture ring should slip off, or possibly thread off. Attach the new aperture ring in the manner you removed it. Replace the mount and screws. Make sure the screws are tightened securely.
If your lens mount does not have screws in it, like the one shown below, then you have the easy style to change.
You will need to look for screws on the side of the aperture ring, like in the picture below. Some lenses have one, some more than one.
Remove the screws that hold on the ring. In some cases the ring may thread of, some it may slip off the back. Pay attention to how it came off, and replace the ring with the new one. Put the screws in and you are done.