See:
I also made some step by step on the French forums… so here goes:
If you have already installed Warzone, MW II and/or MW III, you only need to modify the installation to add BO6… click on the gear-like icon on the right on the Play button and select Modify Installation. You will get, I think, screen #2 of the installation (see below).
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Otherwise…click on Games… then on Activision… then on Call of Duty.
You should now see an Install button… click it.
When you click on the Install button, you should get the 1st screen… with the Install Location:
Then click on Continue… and you get the 2nd screen:
Click on the Start Install button.
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There are also a few Support article that may help you:
Hopefully… that will help !