Does this end date affect all employees?
Hope you don't have a huge payroll
If it affects all employees delete the payrun and reprocess and submit to ATO as amendment.
Check that YTD figures haven't changed.
My understanding with STP is that the YTD figures are what they are interested in so don't sweat the ATO side of things too much - the next payroll reporting will override this one anyway.
Do you pay on a regular basis? if it is weekly or fortnightly you can set that up and the dates will be automatically set and errors won't be made
Hope this is useful
Lisa