how do you change a speed sensor on a 2003 chevy tahoe?

[courtney g]

my speedometer doesnt work on my 2003 chevy tahoe. i've had it over two years now, and its never worked. i bought it from a used car dealership, and i was supposed to bring it back for them to fix it, but never had the chance to do it. i'm guessing its the sensor. the only gauge that works on the dashboard is the temperature gauge and the RPMs. give me ideas of what this problem can be? and i'd like to know how to change the sensor. im going to try that.
or the instrument cluster? maybe? any suggestions would be appreciated!!