Need an instruction how to replace a tail light cover. thanks.

You don't need to remove the rear bumper cover to remove the tail light units. See the link.
Chevrolet Impala 2014 2018 Tail Light Housing Removal

1. Replace your car tail light. You will need working gloves to protect your hand during the process. Of course you will also need a screwdriver, cleaning cloth, wire brush, and a good set of tail light bulbs.

2.Understand that different models of car have different set of tail light covers. Naturally, these covers will require different un-installation procedures. So, determine if you can dismount the cover from the outside or you will have to dismount it from the trunk of the car.

3.Determine how to dismount the tail light cover, carefully detach it from the car. You can use different sets of screwdrivers for this. Be careful not to break the tail lights cover. The important thing is you must be able to expose the bulb in order to extract it.

4.Remove the tail lights cover, unscrew the bulb from its socket. Sometimes you will have to press a little and twist the bulb in order to extract it from the socket.

5.Extract the bulb, bring it to your favorite auto parts and accessories store and find the exact match of your tail light bulb. Be sure to get the correct match because if you use other types of bulb, your tail lights may not work properly.

6.Reinstall the tail light bulb, clean the aluminum casing of the tail light. Wipe the socket and wiring with a dry cloth. You can use wire brush if there are corrosion around the tail light casing.

7.Put the good bulb on the socket. You might still need to apply a little pressure on the socket before you can install it fully.

8.Reinstall the tail light bulb, replace the cover of the tail lights and secure it tightly.

9.Test to make sure your new tail lights are working. Start the engine and apply the brakes. You can ask someone to see if the tail lights are really working.

resource: http://www.wikihow.com/Fix-Car-Tail-Lights

I was looking for the instructions for your tail light removal and came across a detailed description on how to access the tail lights. Here it is:-

http://www.paulstravelpictures.com/Ford-Edge-Tail-Light-Bulbs-Replacement-Guide/index.html

You need to remove the trim from the trunk area that covers the lamp assembly that you are after,then remove the plastic nuts from it and remove the lamp assembly.Now all you need to do is to turn the socket and remove it,pull in the tabs on the bulb and pull it out.Installation is reversed for this removal procedure.

Entire assembly must be changed.

Inside your trunk on either side of your tailgate are panels that allow you access to your taillights....I'll add more later.

If you look inside the trunk, you will need to undo the panel inside trunk to gain access to the bolts that mount the tail light assy's.

Hiya,Open your tailgate and you will see 2 plastic plugs on the light unit these are covering the 2 torx screws which are retaining the lamp unit remove these screws with the SAAB toolkit and you will be able to slide the unit out to replace your faulty bulb.

left tail light is not real. Have a look under boomper. Ther is no kable. Is only one tile light in most of cars.

Here's a great how to video that will also show you what you need to do to change them.