Anything less than 2.5 gpm (gallons per minute) counts. There should be a label on the showerhead. If you can't find one, you can always rely on the saucepan method. Take a saucepan of known volume (4 quarts to a gallon), and run it under the shower (on cold!) to see how many times it fills up in a minute.

The lower the better. If you can find one on Craigslist or at a recycled building materials center, good. If not, they can be found at most hardware and home furnishing stores.

This simply involves unscrewing the old head and screwing the new one on. It may require a wrench and some elbow grease, but it shouldn't be too involved of a process.

Either save the old showerhead as a backup or dispose of it in a way that ensures the materials will be reused. Try your best to recycle the material if it was a particularly inefficient showerhead, or donate it to a recycled building materials center.