17 R5, key is broken, how to get new key?

Contact Dell;  they'll arrange a warranty replacement of the keyboard (Dell does not sell individual key caps - the entire keyboard will be replaced).

Hi @vietnguyen8918  thank you for sharing image. The adhesive has failed. If your warranty has expired, a quick fix would be to carefully remove residue, apply a small drop of cyanoacrylate adhesive (Super Glue, etc.) and ensure cap is reassembled exactly where it should be. Cyanoacrylate works when moisture detected, so when ready, breath on it to instantly cure.