Hi Petra,
in general, such a question would maybe fit better to the SAP Transportation Management SCN group.
I do not know if there is an action (which I doubt) or a helper, but normally the distance is set per stage and then it gets automatically summed up to the total distance. So one possibility would be to change the field DISTANCE_KM of the STOP_SUCCESOR of the referred stage, the total distance will then be adapted automatically.
Best regards, Daniel