replace new record to existing json string

Hi guys,
I have one Json string like this
{ "ReportFilters": [{"groupid":"ef9051e5-def4-4685-9bff-b6b88002b6d8","filterdim":"[Business Date].[Date Hierarchy]","reportdim":"[Business Date].[Date Hierarchy]","customfiltervalue":"[Month].Month-43:[Month].Month-40,[Month].Month-36:Month-30","dateformat":"0","firstdayofmonthcheck":"0","dateorder":"ASC"}]}
now i want add my values in customfiltervalue and reportdim 
like this
customfiltervalue =[Month].Month-43:[Month].Month-40,[Month].Month-35:Month-20;
reportdim= [Business Date].[Date];
after adding json value will come like this
{ "ReportFilters": [{"groupid":"ef9051e5-def4-4685-9bff-b6b88002b6d8","filterdim":"[Business Date].[Date Hierarchy]","repor ...