To do define the Forest Level, place a file named
firest_level.json straight in the forest folder.
This file should be a simple json object with a single property: Level.
Supported values: 2008R2, 2012, 2012R2 or 2016
It is impossible to downgrade a forest functional level, so even if you define a low level, it will not be applied if the domain is already at a higher level.
It is impossible to upgrade the forest to a level not yet reached by all member domains. In order to upgrade the forest functional level, be sure to first update every single domain.