How to modify the number of registrants in backend - by: anton fluekiger

Simple question:
A groupe of 10 people made a reservation as a groupe. 5 days before the event the leader of the groupe is writing an email that 4 persons are not able to attend the event. How to update the new size of the groupe in backend in order to release the available places for new registrants.
The size of the groupe is the only thing I can not change in Backend under events / registrants and then clicking on the name.
Is there a way to do this without delete and re-register the groupe?

Thanks