As you've already found out, the global variables can be accessed through ORK.Game.Variables, there are different Set functions to set a key to a value (i.e. bool, float, string, Vector3 values). E.g.:
ORK.Game.Variables.Set(key, value);
Depending on the type of the variable you want to change/check, there are also some specialized functions (e.g. changing the value of a float variable using an operator).
You can find all available functions in the API.