Depending on the amount of damage (non-structural), it's not worth the hassle unless you pay someone to do it. However, it the mood strikes you anyway, you technically have to remove the siding, replace the channel and reinstall the siding. If there is anyway to get a pry bar behind the back side of the channel, you might get lucky and pry the channel out and twist a new one in, securing the channel then becomes the challenge. Perhaps some construction adhesive to the wall surface of the channel will do the trick.