The window channel can be purchased in flexible style. Using this style, I was able to side the window all the way open and remove the existing channel (rigid or flex) by removing the small screws in the bottom of the channel and yanking it out with vice grips. The flexible replacement was slid into place above and under the glass starting with the bottom slot and the glass all the way opened again. Sealant can be laid into the track all the way from the point when the open glass is to the other end of the open slot. For the bottom channel, place the channel into the slot, and holding the glass up a bit, slide it under the glass until the glass can be pulled over the new channel. Keep pushing the channel until it reaches the far end of the slot - this is easy if the upper channel has been pulled out. You can pull up the flexible channel to sqeeze a bit of your favorite sealant into the slot if needed.
Another method I used on an externally wood framed window was to open the slider two inches and using a plunge cutter, cut slots on the top horizontal wood frame immediately ahead or and behind the slider clear down to the cabin side. Then the wooden frame between the slots was removed using the plunge cutter to cut the jiont between the frame and cabin side after removing any screws. Now the slider is simply levered out of the frame from the top, and both old top and bottom channels are removed. The bottom channel is replaced, and the window slider is put back into its place. Since the upper channel really does not need to be sealed into place (assuming its slot is sealed up), you can replace the frame cutout and finese the new flex channel into place starting with the slider open. The upper channel with be held in place with a few small screw through the uppermost surface.
