First, select the references you would like to export by clicking each or use shift or ctrl (cmnd for macOS users) to select/deselect multiple references at a time. You may also use this same method to export your entire library.
Click the Output Styles drop down and choose Select Another Style...
(if BibTex Export is already appears skip to "Select the files..." below)
From the Choose A Style menu, find and select BibTex Export and click Choose
Select the files you wish to export, or your entire library, then click File then Export
Save file as type: XML Be sure Output style is set to BibTeX Export
Be sure to check the Export Selected Reference checkbox to reflect whether you want to export your entire library of refrences or only selected references