You can take any number of trains on a travel day. Also in your country of residence.
But if I understand correctly, you are now in a night train to Offenburg, where you will change to Strasbourg? Which night train is that and where did you board it?
In any case, you can't cancel a travel day after it has started. But how many inbound/outbounds have you used now and how many should it be? And how many travel days? You can see that under My Pass.
If you would like to cancel the travel day of 24 March (and the inbound/outbound linked with it) because you'd only be using it from Offenburg to Strasbourg (I'm making a guess here), then you could contact customer service. Do remove the journeys for 24 March from My Pass and buy your ticket to Kehl Grenze or Krimmeri-Meinau (I'm not sure which one is cheaper) but you won't have a response until after you've arrived in Strasbourg, so it's a bit of a gamble.
If you only want to cancel the inbound/outbound, you'll also need customer service.