The best known (and IMO most full featured) eBook converter is Calibre.
This page says that
calibre supports the conversion of many input formats to many output
formats. It can convert every input format in the following list, to
every output format.
Input Formats: AZW, AZW3, AZW4, CBZ, CBR, CBC, CHM, DJVU, DOCX, EPUB,
FB2, FBZ, HTML, HTMLZ, LIT, LRF, MOBI, ODT, PDF, PRC, PDB, PML, RB,
RTF, SNB, TCR, TXT, TXTZ
Output Formats: AZW3, EPUB, DOCX, FB2, HTMLZ, OEB, LIT, LRF, MOBI,
PDB, PMLZ, RB, PDF, RTF, SNB, TCR, TXT, TXTZ, ZIP
Calibre is stable (10 years already), free and runs on Windows, Linux & osx.