If the second train is a train without mandatory reservations it's relatively easy : simply add the later train to the app and hop on it.

If it's a train with mandatory reservations speak to staff as soon as possible. They'll redirect you to the next available train (in 1st/2nd class, doesn't matter) while issuing new reservations free of charge. That's the ideal situation but you may need to visit an info point/ticket counter at the station.

If the delay is big (often more than 60 min) you may be entitled to compensation. If it is not possible to complete the journey, a night accommodation or a taxi can be offered. It works on a case per case basis and some companies are more helpful than others...