The easiest way without getting too complicated would be to do a Save As,
go back into the wizard, change the gal/person/day value, run the wizard,
and see what the new source flow GPM value should be on your Domestic Water
Loop. This should be located under the Process Loads tab. Now go back to
your original model and update the flow value.

Outside of that method, you can create a spreadsheet that totals the number
of full load hours in the schedule you're using for the DHW process flow
and calculate the total gallons per day based on that. Adjust the GPM as
necessary to achieve the total gallons number you're shooting for.

Robby Oylear, PE, LEED AP