How to amend activity dates in a network?

Hiya,

I'd like to amend the dates that already exist in a network activity i.e. The 'Act' dates for 'Start' and 'Fin':

I've tried using FM BAPI_NETWORK_CONF_ADD and passed my new dates into that FM, and then COMMIT WORK, but unfortunately that didn't work.

Would anyone have any better ideas?

Thanks in advance!