Hi Dongkai,
Seems the question is not relevant for planning application.
I think the question should be: How to use the method CL_RSR_PARA=>CHANGE to change the variant value stored in table RSRPARAMETRIZA.
This method is used to cooperate the variable via interface IF_RSR_VAR_RUNTIME, so it involves some variable class object. Maybe you can set the relevant break-point at the relevant part to debug and learn how the variant value is saved into table RSRPARAMETRIZA and how to process the change action.
If you would like to really understand this logic, maybe you can try posting the question at SAP Business Explorer (SAP BEx)
to see whether there is relevant expert can answer this question or not.
By the way, can you share some information about why you want to change the parameter for planning sequence. For this question, maybe we can provide the suggestion to help you find a suitable solution.
Best Regards,
Sheldon.