Turn off power to the indoor unit. This may be more than 1 breaker.
Write down the terminal = wire combination in use with the old stat.
Wire the new multistage heat pump stat using the same color code that the old stat used.
Restore power and configure for heatpump operation with electric strips if this is what you have.. (option 170 = 7 with many models)
Configure the O/B terminal to be B energize to heat. (option 190 = 1 with many models)
How to wire a Honeywell thermostat to a Rheem heat pump with aux heat?
Read the directions.
__________________ Ren: Now listen, Cadet. I've got a job for you. See this button? Ren: Don't touch it! It's the History Eraser button, you fool! Stimpy: So what'll happen? Ren: That's just it. We don't know. Maybe something bad, maybe something good. I guess we'll never know, 'cause you're going to guard it. You won't touch it, will you?