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If there are parameters that needs to be passed to the subreport then you would click on your subreport in the main report and under properties, click the subreport tab. At the bottom you will find the button to Edit Parameters. When you press the add button you will be able to search for the parameters created in the subreport and map it to value in the main report.
Note: If you make changes to the subreport and you don't see the changes in the main report then delete the subreport.jasper file.