Originally Posted by JSWolf
What I do is unzip the ePub, and then make the changes and put back just the files that I've changed. Works quite easily.
If the mimetype file is not the first file and/or is not uncompressed, your epub might not work in all epub-supporting software. Calibre, ADE and Sigil can still handle this, but your epub is still non-conformant and may not work elsewhere.