Change staged dunning level for incorrect runs

In the past we had 2 incorrect dunning runs which resulted in wrong letters being send out. Unfortunately the runs were ran as 'final' instead of 'draft'. Now we are looking for ways to correct, reverse or delete the incorrect runs to start form the beginning. Does anyone have tips on how to do this?

We know that in the 'collections' form it is possible to change the staged dunning level to '0' but it seems that there is more to it.

Thanks