I removed two very old round Honeywell mercury thermostats, I'm replacing them with Honeywell digital round thermostats.
The instructions are easy to follow but I encountered two problems, 1) both wires are black and 2) the wires are too short and will not reach the screws in the mounting plate.
So, how can I tell which wire is red or white? And how do I extend the length of the wires?

Thanks

how do I extend the length of the wires?

Add more wire

So, how can I tell which wire is red or white

It will not make a difference since there are only 2 wires to start with.

I would read the directions VERY carefully to make sure that new thermostat
Only requires two wires to operate.