Next open up the Content Browser (Window > Content Browser or hit Shift + F8 on the keyboard) and dock it somewhere next to the Attribute Manager. Navigate to your folder of HDRs and simply click and drag the .hdr file to the image slot of your choosing. If you’ve got your renderer’s Live Viewer enabled, you should see the results instantaneously as you swap out .hdr files.
Great thing about this approach is that you can use this method with any 3rd party renderer and any HDR image that lives on your drive.