You can change it be editing the JF_Avro_GNLU.cfg file
Located at .../LocalState/packages/justflight-aircraft-rj/work/JustFlight.
Remove the line below ...
WeightUnit=KGS
It will default to LBS on the next aircraft load.
But we will update to change it on the fly or synch with the EFB.
Regards Ernie.