I have a Lennox heater model G1205E-737-2 which stopped working, I was told that if filter was dirty it could automatically shut heater down. I found filter was dirty and replaced but was informed that I must press reset button on unit. I cannot find a reset button anywhere on furnace, does anyone know where it's located?

Your model number exactly as posted does not bring up a unit. Check the manual below to see if it is your furnace. It is for a G12 series unit which might be what you have.

http://www.lennox.com/pdfs/manuals/Lennox%20G12%20Manual.pdf