If you want to go back to grid view from the loupe view, then press "g" key.
Or, if you mean that is showing magnified view in the loupe view itself for each image then, you need to click on "FIT" in the navigator panel on top left.
Thank you Chetna, that was very helpful! Is there a way to run an automatic
slide show in LR?
Again, thanks for getting back to me so quickly!
On Tuesday, March 29, 2016, Chetna Srivastava <email@example.com>
You can go to Window menu and choose Impromptu slideshow or use shortcut Control+Enter/Command+Return