I also searched a long time but I found it.
1. Go in the admin panel to “Plugins” and then on “Editor”.
2. Select the “Event calender” Plugin.
3. search with “ctrl+f” for “Upcoming” with big “U” case sensitive
4. you should get 2 results. Change both in whatever you want, e.g. “N?chste Termine”
5. save it.
Now it should be changed.
