Unscrew the nut that retains the pulley. If it's too tight to wrench off with handtools, take the alternator to a tire shop and have them use an impact wrench. After you've removed the pulley, take off the fan and the spacer. The rotor should slide right off. If it's rusted on tight, hold the alternator and give the shaft a few taps with a hammer. Also, before you reinstall the shaft, apply a thin layer of all-purpose oil (such as 3-In-One) to the threads and only to the part where the front bearing goes through. The rear of the shaft can be wiped down with a rag or electrical contact cleaner.