Flog, welcome to the forum.
Do a search in the forum - there's a thread on how to replace the guides without lifting the top above the bottom half. Basically, you take the tracks off the bottom half and that gives you additional room (you can "tilt" the top out) to unscrew the old guides and screw in the new ones. I think you do it with a right angled screwdriver.
Hi-Lo 1707T - Tire Minder TPMS on Tow Vehicle and Trailer, 300W Solar Battery Charger, Equal-i-zer WDH
2014 F150 Platinum 4x4 3.5L EcoBoost SCrew