I want to change my old thermostat for they RTH5100b and the old one got only two wires white and black and no letter.
Where the wires goes on the new one

If there is only two wires on your tsat put the wires on rh(should be a jumper on Rh&Rc( and the other on w and if you have a/c if you have a main control switch on the furnace or a/h that switch from ht or a/c your OK good luck Mikie D