If this is factory AC, I did it without dropping any major parts. This is what I started with:
Under the unit there are little screws holding the end-brackets in place
Here is one of the brackets:
I saw that the first one rotated too much, so I put in some black adhesive-backed felt, cut in a strip to keep it in position. Felt can be purchased at JoAnn fabrics; Hobby Lobby, etc.
Have an awl ready, or something to help you re-align the holes. Loosen the bracket at both ends. I started at the far left louver and then left the right side bracket looser to help get the next louver in. Tighten the left bracket on that one, again, leave the right side bracket loose to insert the next louver, and so forth. The last bracket is a bit fiddly, you have to have some play in the brackets on both sides to fit it.
Project all done. Probably a 20 minute to half-hour job.
Here is the link to my original post on this topic: