Limiting Date Selections by Editing Record from Selected Dat

I am creating a registration form with a lookup field that grabs available dates from another form. The dates are currently only used if their Taken field = no.
Upon success, I would like to take the date that was selected and edit its record, specifically the "Taken" field from "no" to "yes", so that future registrants cannot pick this date. How can I fetch just the record that was selected in the date lookup field and dynamically edit its fields in the other form?