Citations are usually exported between RIS, various bibliography formats from TEX/BibTex, Endnote formats or even XML. You need to use a dedicated bibliographic or text referencing software like the Reference Manager from Thomson Reuters.
Convert Research Information Systems citation to BibTeX document.
We have found 2 software records in our database eligible for .ris to .bibtex file format conversion.
Microsoft Windows software - convert ris to bibtex on Windows
Bibliographic reference and citation management software
Convert from ris file format
The tables with software actions are a good pointer for what a certain program does with various file types and often may give users a good hint how to perform certain file conversion, for example the above-mentioned ris to bibtex. However, it is not perfect and sometimes can show results which are not really usable because of how certain programs work with files and the possible conversion is thus actually not possible at all.
Apple macOS / Mac OS X software - convert ris to bibtex on OS X
A bibliography manager for Mac