Check the late appeal rule
Find the university's rule on extending a deadline or accepting a late appeal. The decision letter and appeal policy may explain where to send a request and what evidence it requires.
Give a dated explanation
State the deadline, when you became aware of it, what prevented submission and when the obstacle ended. Include dated supporting material where possible. If disability, illness or missing notification is relevant, explain the connection to the delay.
Submit promptly
Contact the appeals office without waiting for a perfect pack. Ask whether it will consider a late appeal, provide the available documents and retain the reply. If a late submission is refused, ask for the reasons and any further internal route.