Here's how to make adjustments to the pop-up modal.
Open the Navigator panel. Find the Symbol named "Appointment Modal", double click it to open it.
Once you are inside the symbol, on the Style panel change the Display setting to Flex.
With the Pop-up modal now visible you can edit it how you see fit.
To close the modal, simple select the parent container named Appoinment Modal Wrapper and change the display to None.
You're all done.