If you have set the availability for a particular interviewer, and that interviewer has now become unavailable or needs to change that time, you will need to delete their availability block and re-add it with the correct date and time.
Click on the 'Interviews' tab in the main menu and click the green 'Manage availability' button on the right-hand side.
Navigate to the interviewer availability that you need to delete and click on it. You will see a pop-up box with a 'Delete' option.
Click 'Delete' and that availability will disappear from the calendar. You can then re-add the interviewer's availability on the correct date and at the correct time.
If you had sent invitations to applicants at the deleted time or if they had confirmed their interview, you will be prompted to either cancel their interview and offer a new time or to cancel their interview and revoke their invitation.