- Edit only the odt file. Use styles & headings.
- Do an extra Save As a docx file
- Convert docx to epub2 yourself (recommended by Amazon). Use either Calibre or Sigil. Do not use Indesign for other than PDFs for paper.
- Upload the epub2 and cover separately to Amazon KDP. Same files work for Google Play books/Playstore and Smashwords (who send it on to Apple, Scribd, Barnes & Noble, and Kobo).
I've used Sigil and it's recommended if you are doing anything that doesn't work automatically for docx -> epub2 in Calibre. You CAN edit CSS & HTML in Calibre editor too, but KISS ebooks work perfectly (without CSS or HTML edit) if you have the MS Word or LO Writer styles & headings done properly.
Only use freely usable fonts, embed and subset. Check that the ebook is OK on old kindle (Kf7/mobi) and newer kindle (azw3 or kF8 mobi or dual mobi) when default Kindle fonts are used. You can't upload Kindle versions to anywhere except Smashwords (dual mobi best for that), they are only for local testing.
Only edit the odt, never the docx. The docx is only for making the epub2, it works less well to upload that to KDP than an epub2.
Amazon then make mobi, azw3 and kfx for customers according to which Kindle or app they use and if Download via PC or "whispernet" direct delivery (2G/3G/4G or WiFi).