Rescheduling or canceling individual interview can happen in the Application view of the specific applicant. This procedure assumes, that an applicant is still at the interview step.
Choose Summary tab under the application data. Under Activities heading, click "Reschedule or cancel".
Message tab will be opened with the rescheduling message. Click Offer interview times.
Calendar popup appears, where time slot, confirmed by an applicant appears in red, offered but not confirmed appears in purple (calendar opens today, changing month may be needed). Clicking the red interview slot will effectively cancel the interview. After that, new interview times can be chosen or added.
Insert link in the message (button at the bottom of the calendar popup), review and send the message.
Please, note, that clicking on Red slot causes interview to be immediately canceled, so if done by mistake, it is recommended to send a new letter, possibly adding more times.
Read more about scheduling interviews here.