Change it in the app, in Me under «Exam date», or on licento.com in your account, and your plan is rescheduled around the new date.
Your plan works towards your exam date, and nothing is scheduled after it. It spreads the questions you have not met yet over the weeks up to two weeks before the exam, and keeps those last two weeks for review.
The date cannot be in the past, nor too far ahead: if it is, you are told the latest date you can give. In the app, setting the date needs a connection.
If your exam date has already passed, say how your exam went instead: see «My exam was postponed» or «I failed: what now?».