Hi urmas,
In case you really want to modify that process only without creating a new copy of that, then either you have to wait till all the initiated instances of that process is completed, or you have to cancel all of them.
Then you have to delete all those completed/cancelled instances so that you can again modify the Process definition.
But however, a lot of risk is involved in this approach as you have to be very sure that you are not cancelling any processes that needs to be completed or something like that. If you are sure of that then I guess it is fine. Hope this helps.
Thanks,
Abhisek Dhar