There a couple of ways to navigate pages in Launchpad. If you are using a Apple Magic Mouse or a trackpad, swipe with two fingers from right-to-left to move foreword one page or left-to-right to move backward one page.
If you are using a keyboard, hold down the ⌘ key and then press the left or right arrow key to move backward or forward one page, respectively.
You can also click on one of the white dots on the bottom of the page. The dot furthers the left corresponds to the first page, the next dot corresponds to the second page, and so on.