This will be a brief how-to on replacing a bedside panel.
1) Remove tailgate, tail lamp, bumper, and any other items needed to access the bedside.
2) Remove the bed bolts that mount the bed to the chassis, slide the bed back afterwards to access the spot welds on the front of the bed.
3) Grind all factory spot welds holding the panel to the bed assembly. (These are commonly located in the front of the bed, the wheel well, in the tail light pocket, and on the top and bottom side of the panel.)
4) Once all the welds have been ground you can use your preferred tool to go around where you have ground all the spot welds and pop them loose. The panel should practically fall off after this step.
5) Replace the bedside with your supplied replacement part just doing these steps in opposite order. Depending on the manufacturer you can either weld or glue the panel back on or both.
Tips: Keep and eye out for foam on the top rail of bedsides. Using seam sealer over your welds after they have been ground makes for a clean finish. Having patience also helps.