Train travel is like taking the subway. Only over longer distances and in nicer trains. It is not like air travel. But is just: get to the station, find a train, get on, find a seat.

In the past tickets were based purely on distance travelled. I bought tickets from my home station to places far away in Europe without even knowing what trains we would travel on, as there was no internet there. We just left, and figured it out on the way. We had bought a route, not a place on a train after all.

Things are different now. Many railways now require reservations on long distance trains. Other railways give you discounts over the standard fare if you commit to taking particular (often less used) trains. German railways does that. And German railways have a rule that if the trains your ticket is linked too are no longer running your ticket falls back to being a flexible ticket.

So you now have a ticket that allows you to take any train on this route, within the days of validity (have a look at the validity date which should be at the top, and which can be up to four days).

So just look up a convenient schedule for you on www.bahn.de and use that.

The railways assume that you already know that (99,9% of their customers are peopel who travel by train regularly) and thus they do not always communicate what you should do in case of changes in a clear way.