Through Sketchup, via the Ruby Console:

sketchup_model = Sketchup.active_model
model = sketchup_model.model_interface.openstudio_model
gr = model.getSiteGroundReflectance()
gr.setJanuaryGroundReflectance(0.2)
... rinse and repeat...

Here is the documentation for the SiteGroundReflectance class (not the "Site_GroundReflectancefields" class, which is something different).

The setJanuaryGroundReflectance (etc.) methods should work to set the ground reflectance by month.