1. Go to References > Export or Tools > Export, depending on your version.
2. Select All References or a specific folder of references you want to export.
3. Under "Export Formats" select BibTex and XML. XML will be your archival copy, and BibTex will be the secondary copy, one which you can import into other reference managers or use with LaTex software.
4. Click "Export to Text" and save.