what should be default install directory be ?
If you did not change it
this is the default path for the game
C:\ProgramFiles(x86)\WorldofWarcraft\_retail_
This is the install directory for the World of Warcraft executable and the data directories for the retail version. PTR or beta versions would be installed in separate directories along side _retail_. The Battle\.net launcher executable is actually in the “WorldofWarcraft” folder next to the folder of the retail software.
This change in directory structure (moving the WoW program into a subdirectory, separating where the WoW executable and the bnet launcher are stored) has caused issues for external programs that needed to understand the directory structure (like video driver optimizers or the WowHead client).
I noticed the WoW launcher forces you to create a “world of warcraft” subdirectory. I think this is a recent change, as i had installed wow in my own directory path. If you find that the launcher can’t find wow any more, rename your custom wow directory to “World of Warcraft” then commence installation. The launcher should see the existing installation, and then it will change from installation to “update”.