Are they worn? If not, regrease, and put on new bearings.
If they are, replace the headset. They are an inteference fit; use a screwdriver to bash them out from the inside, starting on one side, then the other. You will also need to remove the fork crown.
This is something where it is really better to have someone show you the job the first time you do it, especially if your frame is aluminium. It's nothing to be scared of, but if you get it wrong you can wreck the frame.