Hi Arman,
When you change the weather file on the Location and Resource page, it does not change any other inputs in the case, including load data, costs, incentives, or any other inputs.
When you provide kW time series load data, the peak load is the maximum kW value of the load for each month, and the total monthly load in kW is the sum of the kW values over the month. It is not possible change the peak independently of the monthly total.
The webinar on Residential and Commercial Electricity Rates has more information about working with load data in SAM, and the short How-to video shows how to import load data:
sam.nrel.gov/financial-models/residential-and-commercial
Best regards,
Paul.