Lester Says " Follow the instalation instruction carfully when you are installing your new thermostat. But it is very easy to upgrade your current thermostat with a new energy saving model. Some easy thing you should know is when the terminals are called out with a letter or letters those leter usaly represent a color GR means Green R Red W White B Black"
Check your system to make sure that your thermostat is of the the same type.
Use photos to ID your replacement parts. Lester Says "If you left click your mouse you can download the photo and use your photo software to size to full size to to make sure the part is what you need.