Take the Piccadilly Underground line to the Knightsbridge line. There are two exits in the station - when Harrods is open you can take the exit which lets out directly next to the huge store. However, we were visiting on Sunday, when the store is closed. Let me just say that Sunday isn't the easiest day to visit the city - many things are closed. However, crowds do seem to be smaller.
The area around Harrod's is full of upscale shops. Some of them are open on Sundays, but most are not.
The store is interesting to see, even if it is closed. It's enormous and very impressive. If you do get the chance, see it at night when all of the hundreds of lights on the facade are lit up.