Both are methods of an action. Hands up is an 'physical' action to take place. Heads up is an 'alert' of an action that you should be weary of.

Hands up -> put your hands up (physically) Heads up -> Attention to something / be careful. :)

Hands up 🙌🏽 Heads up warning ⚠️

Heads up: Warning So in context when someone says you should have given me a heads up about something they simply mean tell them prior.