To replace the bottom weatherstrip on a garage door, first pull the door's emergency cord to disconnect the door from the automatic opener. Manually raise the door until its bottom is just below the curve in the door's track. Ensure it remains stable by placing a C clamp over both the roller and the track. Many weatherstrips are secured by closing the grooves at the ends. If that is the case, use a flat-head screwdriver to open the grooves.
Once the grooves are open, slide the weatherstrip out. If it does not feed easily, use a box cutter to slice the strip down the middle and forcefully pull each side from the groove. Use lubricant to clean the grooves. Fold the new weatherstrip into a U, and feed it into both grooves simultaneously.
Continue feeding until approximately 3 inches hang over the opposite side of the door. Fold this excess into the U formation until it is flush with the door. Using a box cutter, trim the opposing side to 3 inches and fold accordingly. Use a pair of pliers, and carefully crimp both ends of the grooves in order to secure the weatherstrip.
Take the C clamp from the track and roller, and close the garage door. Pull the emergency cord again to reactivate the automatic opener, and test to see if the door opens and closes smoothly.