The other day at the office I had to delete a floor from one of my Revit models. Yes, I know, the horror! However, just follow this quick tutorial, and before you know it, you will just be deleting floors from all of your projects for fun! Disclaimer: I do not endorse making buildings smaller.
I came to the realization that the best practice to do this is to just get rid of a typical floor within the center of the building. For example, if you have a 50 story building, and floors 10 – 40 are typical, pick any one of those floors to get rid of. The method I do not recommend is deleting the roof level and renaming level 50 to roof. This will lead you down a very scary path of un-referenced walls, roofs and other things that need a reference (which, by the way is everything).
Take a loot at our starting example below:
What you need to do first is to delete everything on the floor you wish to get rid of. In our example let’s delete the 6th floor. Remove all of the windows, doors and other various objects that are hosted to level 6. You also want to go to the plan view and delete all of the interior objects that are hosted to level 6 (interior partitions, furniture, fixtures, etc.). It is very important to complete this step before removing the actual level to prevent any overlapping objects or floaters. Go to the elevation view, select level 6 (or the level you wish to remove) and hit delete. Your elevation should look like the example below:

Select all of the levels above where 6 used to be, and move them down to where level 6 was back in the day. Our imaginary project has floor-to-floor heights of 10′-0″, so I moved everything down 10′-0″. See the example below:

Rename all of your levels accordingly. Consider that floor removed.
Keep in mind, if you have to add a floor to a large building, you can do the same thing, but just in reverse. Insert the new level in the center of the building, move the levels you need up, and rename accordingly.
Please note that this example really only works if the building is just too tall and it needs to be shorter. If your client (or boss) really hates the top floor of your building and wants to get rid of it altogether, then you can remove the Roof level and rename the next floor to Roof.
UPDATE! If for some reason you get an error message stating that “Elements will be deleted”, check to see if you have Disjoin checked in the options bar. If you do, un-check it!