I need to replace a robert shaw 9420 thermostat wit a programmable one. What will work

Thermostats are chosen by The type of system you have in your home and the features you want to have on the thermostat. So lets start over so you can supply the proper information.

What kind of a system do you have?
What kind of features would you like to have on your new thermostat?

Read below link so we can serve you better.

https://www.askmehelpdesk.com/heating-air-conditioning/read-heating-air-conditioning-faq-hvac1000-284131.html