Sooner or later your reverse lights will need replacement.  Normally this should be an easy task.  However in the Impala, it is a little more involved.

You will need bulb number 921.  I believe they come in pairs.  I recommend replacing both, even if one is still good.  Chances are that the other bulb will fail pretty soon and no sense doing this more often than you have to.

Before starting, look closely at the gap between the top of the reverse light assembly and the trunk.  There is a soft black weatherstrip that prevents water from getting behind the lights.  When you replace the assembly, you need to ensure that there is no gap here.  Open your trunk and remove the nine wing nuts.  Some are easier to remove than others.
After you remove all the wing nuts, hold the assembly to the car and bring the lid down.  The trunk latch does not have to engage.  Actually, it's better if it does not.  If you attempt to slam the lid down, you might lose control of the assembly and drop it.  Pull the assembly straight out.  BE CAREFUL!  There are two concerns here.  First, there isn't much slack in the harness.  Second, the wing nut studs are sharp.  They can scratch the paint on the trunk lid behind the lights.  Here you can see the two light sockets.
To remove the socket (1), press in the tab, turn the socket counterclockwise, and pull it out.  Ok, I mentioned that the studs (2) were sharp.  The bases of these studs are sharp also.  The white, round rings in the black paint in the background is not paint.  That's where the bases cut into the paint.  I don't know why Chevy didn't put a plastic washer back there to protect the paint.
If you don't feel like trying to replace bulbs while attempting to hold the light assembly, you can remove the harness.  A piece of tape holds it in place.  Remove the tape carefully then take the harness out of the posts that hold it in place.  This is a photo of the trunk lid with the light assembly removed.
This is the back of the light assembly.  The eight dots are the studs for the wing nuts.  The ninth stud is obscured by the glare.
Pull the bulb straight out.  Do not twist it.  Replace it with a new bulb.  Now put everything back together.  If you removed the harness, make sure the wires go back into place and are not pinched.  Put the assembly back into place and watch out for the sharp ends of the studs.  Loosely attach wing nuts on each end.  Aren't you glad you didn't latch the trunk.  Otherwise you would have to get the keys and try to open the trunk.  Right about now you realize that the keys are now IN the trunk...  Now is your chance to align the assembly so there is no gap between the weatherstrip and the trunk lid.  Tighten the wing nuts and install the rest.