FenestrationSurface:Detailed does have a multiplier field that could be used to increase or decrease the effective window area, but as mentioned in the EnergyPlus documentation for FenestrationSurface:Detailed this should be done with caution, as depending on the scenario itmay scenario, may produce odd results, warnings, or simulation errors, this isn't a very large model, so changing the geometry by hand instead should also be very easy, unless this is a test for something larger.