May I ask why you believe you need to replace the water line within the confines of the freezer door? This is not a common refrigerator repair. I've had many customers think they have a clog of some sort in the water line, but that really is impossible, unless of course the blockage is ice. But if that is the case, replacing the line wont resolve the issue, as the ice will return until you determine why its building up in the first place.
If you are not getting water out the door dispenser, your most likely problem is either the water valve solenoid is no longer opening to allow water flow past the valve, or there is an electrical issue in the dispenser system itself, or to the connection at the valve.

If you are certain you need to replace the water line, its not an easy job. Before you even attempt anything, you should peel the door gasket/seal back, to look underneath gasket to make sure you can do any repairs. If there are several screws under the gasket, proceed further, if not, stop here, as you will not be able to disassemble the door to get to the water line. your door is glued together, and you will highly regret attempting on your own.

you will have to remove the door entirely from the fridge, including the electrical connection and the water line connection at the bottom. Once the door is removed and laying liner side up on the floor, unscrew and remove all the screws, remove the liner and gasket together, then remove but dont throw out all the fiberglass insulation around the dispenser, You will eventually see the water line fed through the dispenser area. carefully remove the water line from the path around dispenser, then pull it out of the door itself entirely. now take your new water line, and from the bottom of the door, feed it up towards the dispenser, through the same path, making sure that the end of the water line is at the exact same depth in the dispenser, leaving more than enough line at the bottom of the door.
Replace all of the insulation previously removed, its best to put it in the same orientation as it was originally. If you dont pack the insulation as it was, it can cause empty pockets of air inside the door, and its in these pockets that the normal ice condensation build up that occurs inside the door, builds up ice, which can surround a portion of the water line, freezing it, rendering your DIY repair job useless. Reattach the gasket and liner, dont overtighten the screws, as you may crack the door liner, which is not a good thing either. You may have to do some back and forth adjustments, loosening and re tightening screws, to make sure the entire length of all four sides form a tight seal when door is closed. Trim the water line to the correct length, and reattach. make sure you replug in electrical connection.

thats your basic procedure.