Dropbox schedule location vs actual drop off location vs interview location

Hi, first time dealing with drop box option. I am getting drop box appointment at Delhi although my preferred consulate location is Mumbai which I selected on DS 160. I believe I can select Delhi but actually will be able to drop off at Mumbai. But my question is, if they call for an interview, where will it be? At Delhi where I got drop box appointment or at Mumbai where I dropped off documents and mentioned on my DS 160 as preferred location? Also If I schedule appointment date and time at Delhi, but later on if Dates open up in Mumbai, can I simply change the appointment location from Delhi to Mumbai?