To export Mendeley citations into a .RIS file, first select the citations that you want to be exported.
Next, click on File and select Export.
Next, make sure that the Save as Type is in a .RIS file format. Once that has been done, click on save and your citations will be exported.
To import .RIS file into EndNote, first click on File followed by Import --> File.
Make sure that the Import Option is Reference Manager (.RIS). Choose the .RIS file that you want to import and the citations will be imported into EndNote.