Legally per the NZTA we cannot edit your logbook or even you once it has been locked. Rest assure you are not the first person to do this so in the event you need to record an unusual situation in your logbook e.g. you made a mistake or have Locked an incorrect entry we suggest that you add a Note and move on to your next Shift.
Have a read about how to add Comments & Notes to your logbook in this article.
Now in the future I recommend you review your logbook before you lock it because you can make changes to your Logbook before you Lock it.
Refer to this article on how to Edit a Logbook entry.