Here are two easy ways to convert RIS to BibTeX format.
The second way also gives you the handy
bib2pdf program that creates a PDF listing of your bibliography directly from the
ris2bib Bibutils setup
apt install bibutils cb2bib echo 'export PATH=$PATH:/usr/share/cb2bib/c2btools' >> ~/.bashrc ris2bib in.ris out.bib