I installed a gas furnace and want to use the old furnace circuit, how do I disconnect the furnace wire correctly to leave in the panel for future use?

Turn off main circuit breaker, remove panel front cover.

Disconnect wires. If this was a 120V circuit, the black wire will be connected to the circuit breaker and the white will be connected to the neutral buss. If this was a 240V circuit both wires will be connected to the circuit breaker.

Put a wire nut on the end of each wire and tuck out of the way. Put front panel back in place. Turn main breaker on.