Originally Posted by fransknorge

You can take any trains you want with your ticket now, on the same day. No need to get a new ticket or anything, just turn up.
If you want seat reservations, you do it on the DB Navigator app, DB website or a brick&mortar DB office. Simply follow the same process as booking but tick on Seat reservations at the beginning of the process. You can do it anytime before the train departure, the sooner the better obviously.