For those that may be getting frustrated with trying to play DUWS-R on a different map here are the details that no one tells you :-D. By default the map name is "Altis" where the other pre-installed map in Arma3 is "Stratis".

TL;DR;

For Steam:
* Go to the location you installed steam (Ex. D:\Program Files (x86)\Steam)
* Open the "userdata folder" (Ex. D:\Program Files (x86)\Steam\userdata)
* Search "*.pbo" or "Dynamic%20Universal%20War%20System%20-%20Rebirth.Altis.pbo"
* Rename the pbo map name portion

Old Approach:
* Go to the location that Arma 3 is installed (Ex. D:\Program Files (x86)\Steam\steamapps\common\Arma 3)
* Open either "Missions" or "MPMissions"
* Rename the pbo map name portion

The FULL story:

In the strictest sense all you need to do is rename the "pbo" file to include the name of the map you wish to play on. I'm not really sure why Bohemia Interactive decided to go this route other than that is what they were comfortable with. Right, so now how the hell do we find the flippin' pbo file? Read on wayward gamer; read on.

Arma missions can be installed manually in it's install directory. There are two folders of concern. The first is "Missions" and the other is "MPMissions". The former for single player missions and the latter for multi-player missions. This is not what steam uses.

For Steam:
* Go to the location you installed steam (Ex. D:\Program Files (x86)\Steam)
* Open the "userdata folder" (Ex. D:\Program Files (x86)\Steam\userdata)
* Search "*.pbo" or "Dynamic%20Universal%20War%20System%20-%20Rebirth.Altis.pbo"

Vola, you now have found the pbo files that steam downloads. There's one issue though that you may have more than one version of the file. I suggest that you copy the "pbo" from this folder and paste it into either the "Missions" or "MPMissions" folders located at the same location that "arma3.exe" is located. For me this is at "D:\Program Files (x86)\Steam\steamapps\common\Arma 3". If the "Missions" and "MPMissions" folders do not exist feel free to create them with the exact capitalization as stated here.

The nice thing about the copy paste approach listed above is that steam will display workshop mods/missions and mods/missions installed directly into the "Missions" and "MPMissions" folders in slightly different trees in the UI. This can make it easier to distinguish what is actually going on.