If you're still under warranty then Dell will send you a new keyboard.
You have to send your old one back to them in the box the new one arrives in.
You don't get a single replacement key, you've got to replace the entire keyboard.
I had the exact same problem with the Page UP key on my Dell laptop. Tried to get a replacement key cap, but Dell will only send you the whole keyboard.
Actually replacing it is pretty weasy. The only delicate part is popping off the plastic cover between the keyboard & the screen, which seems like it's almost going to break as you pry it off. The Dell website has an illustrated step-by-step guide for the procedure.
If you're out of warranty, then you'll have to buy a new keyboard from Dell.