Have you tried Sigil the free ePub Editor.
I'm using Sigil to modify various tags.
Update:
1) Start Sigil, open epub.
2) Select Extra-> Meta Editor or F8
3) Select <More>
4) If no ISBN field -> select <Add Basic>, add ISBN field
5) Modify ISBN-field
6) Save epub
Last edited by SkyDream; 11-26-2010 at 01:23 PM.
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