Modfy and add new features to a Existing Pine script

I need to add functionality to an in-built tradingview indicator. on the in-built VWWAP indicator I want the functionality to change the first day of the week (M,T,W,Th,F), in the weekly VWAP function and to select the day (1-31) of the month for the monthly VWAP. This will basically enable me to choose on which day of the week / month the VWAP starts plotting from and resets each time. This will need modification of the built in VWAP script to include the above functions, not creating a new script.