How can I fix the heater valve on my 1999 Ford Fiesta?

My car interior heater only blows cold air. I suspect that the valve controlling the flow of hot air is defective. How do I replace it......are there any pitfulls or tips rather than me just attempting to take off the old one and replacing it with a working valve. Car is a T reg 1999 3 door 1.25 Fiesta Zetec.