If you want to preview and check your ePoster in a web browser then you can just press the preview button (1) and the ePoster will open in the viewer. What you see here is what your ePoster viewers will see if they are viewing your ePoster in a web browser on a computer or tablet. From here you can click on the tiles (2) and check that it is working as you expected. Once you are finished then just click on the X button (3) to close the viewer and return to the editor.
What you see in the preview is exactly what the conference organising team sees when they check and publish your ePoster. So the preview allows you to be sure the ePoster is working as you want and you know that the conference team will see exactly the same.