In dire emergency -- if you fall into boiling lava, or are attacked by the Ravenous Bugblatter Beast of Teledahn -- you will automatically reach for your linking booklet and touch the "home" page.
When you start, "home" means The Surface. However, you can change your panic link destination to any page in the booklet. Select the page; open the menu (the diamond button in the top right of the "Booklet" panel); select "Set Page as Panic Link".
Note that the menu also has a "Return to Surface" option. If your panic link destination is itself dangerous, The Surface is always available as a last-ditch resort.