"Save" (⌘S) and "Save As..." (⇧⌘S) under the "File" menu will save your bibliography as a BibTeX file.
To save a file with an encoding other than the one it was read in, use the "Export BibTeX With Encoding" command, found in the "Export" sub-menu of the "File" menu, or "Save As..." from the "File" menu (if you want to continue working with the file in a different encoding). Choose the encoding you want from the pop-up button at the bottom of the resulting sheet. For a discussion of the implications of encoding choice, see Character Encodings.