If you're meaning page turning direction instead of writing direction...
There's at least three ways to change page turning direction in Kobo:
1. In "Reading setting -> Page Controls", the third method is chosen. In that case, just choose another method.
2. In opf file,
page-progression-direction="rtl" is set. In that case, remove the property, and make Kobo reparse the book.*
3. Something directly updated content.PageProgressDirection='rtl' in KoboReader.sqlite. There's at least
one hack automatically does this if you install it on your device**. In that case, uninstall the hack and make Kobo reparse the book.*
* Which means you have to delete the book on the device and resend it again. If you delete the book while connecting it to a PC, you have to disconnect and reconnect again before resending.
** Although the script I mentioned here only modifies cbz / pdf / cbr, not epub.