working through the track windows, look for the phillips head screws at the front of the rails, one per slider. carefully remove these as they strip easy. use the correct size driver and sometimes an impact is needed.
i usually just slice the old ones off with a utility razor knife. you can work a punch in at the front and drive the sliders back as well, takeing care not to damage the rails.
once the old sliders approach the track, make sure the track window lines up to allow the slider to pass through it. at that point its a tug-of-war pulling the old one out.
clean the rails and lube them as well as the new sliders and reverse the procedure (i use pledge). when the slider gets close to the tip, use a piece of the old one as a driver to align the screw holes.
you can find "mosabi"'s method of removal using the clutch if you do a search.