On the app you delete the train you missed and add the following one. If you need a new seat reservation speak to staff as soon as possible onboard the train or at the train station otherwise. Get a proof of delay in case.

Have a look at this page, it'll answer most of your questions : https://www.seat61.com/how-to-use-an-interrail-pass.htm

@Juan Manuel Vila Jaén También, si el retraso del tren hace que necesitas compensación, puedes pedirla aquí: https://www.interrail.eu/en/support/delay-compensation