GMC vandura speed sensor

I have a 1988 gmc vandura 3500 mini bus. The engine service light is coming on at times so I had it tested at O'Reilly's and they said it's because of a bad speed sensor. I was wondering if anyone could give info on changing this?

Welcome to AF, post the code for proper help, the speed sensor is in the trans. Does the speedo work?

Thanks for answering. No the speedo doesn't work.