If you're replacing the unit (as opposed to simply servicing it), you will need to knock the cups out of the head tube and the crown race off the fork crown. Make sure your replacement parts match (i.e. ISO vs JIS dimensions, and overall stack height) before you get in too deep. JIS head tube and fork crown seats are easily milled to ISO dimensions, but going the other way can be problematic. If your steer tube is too long for the new headset, adding spacers is quick and easy, but if it's too short, you're SOL except for a new fork.