I am not sure about the 4.6L Mustang but I replaced starters in different cars with no problem.
First of all you have to make sure the battery is disconnected before you do it. Then, you should be able to access the starter from underneath the car by the bellhousing. Disconnect all the wires (take notes about the wires locations). Unscrew all the bolts surrounding the starter and it should come out. After you reinstall it, you might need some shims underneath the starter if it gets noisier when you run the starter.