Typical Thermostat Wiring
FIRST!  Turn the breaker off to the unit(s).  This will prevent any mishaps while dealing with the wiring.

You'll want to make a 'roadmap' of how the thermostat is wired now.  Write down which color wire goes to which lettered terminal.  Your new thermostat will have the exact same terminals lettered the same way except in a very few cases.

Replace each wire to the exact same terminal on the new thermostat and you should have no problem.  Don't be concerned with either thermostat wires that are not connected now or terminals that are not used on the new or old thermostat.  If they were important, they would already be connected.  Disregard them, they are for other applications.

If you removed the old thermostat first without noting the wiring arrangement, things are a little more complicated but far from a big deal.  If this is the case, you can try using the roadmaps below.

This is typical wiring for most units.  If you have a digital thermostat now, you MUST pay particular attention to any wire placed at the C terminal on the thermostat.  (Typically the blue wire).  If you are reinstalling another digital or programmable thermostat you will reuse this wire.  If not, discard this wire and tape off the end to prevent shorting and stick it back in the hole to hide it. 

This is the common from the transformer and you can damage the transformer if this wire comes in contact with any of the others, particularly the red so handle it carefully if you didn't turn off the breaker to the unit(s).
and put the wires on the same terminals on the new one as the old one and you should be safe.  Some technicians just use whatever wires are available and don't follow a standard color coded wiring method.

All information here should be considered conversation.  If you are not familiar with electricity, (though this is only 24 volts), and are uncomfortable with dealing with electricity it would be wise to hire a qualified technician to complete the task.  We accept no liability what so ever for any event that may occur as the result of anyone using this information.
Heat Pump T-Stat Wiring
ClGas/Electric T-Stat Wiringi
When installing a New T-Stat, you may have to move a small, yellow wire
that you will see on the t-stat itself and currently under the 'Y' terminal.  If your unit is a heat pump, you will have to move the little yellow wire over to the 'A' terminal next to it.  This tells the t-stat how to control the indoor blower.  Heat Pumps use a relay to turn the blower on when they start.  Gas and electric heat strips turn the blower on themselves then they get hot enough.   Some
are based on time.

These are common wiring examples.  These are NOT GUARANTEED TO SOLVE YOUR PROBLEM OR NOT CREATE A BIGGER PROBLEM!  If you want TO BE SURE you don't have a problem, call a technician out.  If you feel capable, do it but be prepared to deal with any 'negative' consequences should you short something out.

This information is offered as 'conversation' and not advice.  Scott's A/C & Heating, LLC shall not be held liable for any results from any actions  taken as the result of reading this information.

If you have problems, e-mail me or call the office.
R - Red
G - Green
Y - Yellow
W - White
O - Orange
B - Blue