2002 Mitsubishi Magna Power Window plastic tube casing.

Ok so this is my bfs car. His driver side window will go down no problem but it has trouble going back up. So we took it apart to see what the go was. Turns out that the plastic tube casing that surrounds the cable has snapped and bent. This means that when we are trying to get the window to go up the cable is getting caught on the casing. If we put it down to much we then have to hold the button up while we pull the window up. So my question is how do we replace the plastic tube casing on the cable?