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To change a specific VAR with index IVAR use:
psspy.change_var(IVAR,new_value)
To change VAR number N for a plant model with name xxx use:
psspy.change_plmod_var(ibus,id,'xxx',N, new_value)
There are similar API's for other types of dynamic models, like wind machine, loads, line relays, etc. See the API manual.