Extension ladders consist of two ladders of equal size bracketed together so that you can slide one with respect to the other and change the height. A rope exists on most for adjusting the ladder while it is place. This is attached to the bottom rung of the fixed ladder and passed through a pulley at the top of the movable ladder. This rope is protected by a thimble where it is attached to the bottom rung to protect it from wear. Frayed or damaged ropes can be easily replaced.
Set the ladder on its side.
Remove the old rope by pulling the free end through the pulley and then unlooping it from the bottom rung.
Using pre-looped ladder rope, loop the new rope to the bottom rung, ensuring that the loop is protected by the rope thimble.
Pass the rope through the pulley and down the other side of the ladder. Be sure it has not passed behind any rungs and is free of knots.
Wrap duct tape around the free end of the rope to prevent it from fraying.
Set the ladder against a wall and test the ladder by pulling the rope.