Where is A/C switch? How to turn it off?
That panel does not appear to have a main breaker in it, so that would mean the Main is somewhere else, probably outside next to the meter. In that box, there is probably another large 2 pole breaker to feed the A/C unit. You are allowed to have up to 6 breakers in that service entrance box, so because this panel looks full, when someone installed the A/C, they put the breaker out with the main.
But also, is your A/C a central air system, where is it combined with the heating system? If so, there must be a separate switch in the closet with the HVAC unit, usually just a toggle switch, sometimes with a fuse holder too, either on the wall or sometimes on the side of the furnace (see picture below). That is required for doing exactly this; it is JUST the control power for the furnace/AC unit so when you turn off that switch, your little 24V transformer for the thermostat circuit is off too. For what you want to do, that's all you need.