For added security, you can require a four-digit passcode to be entered any time the Dropbox mobile app is launched. This way, you'll know your Dropbox is safe even if your smartphone or tablet falls into the wrong hands.
From now on, every time you launch or return to the Dropbox app, you'll be required to enter your passcode. Your device will be signed out from Dropbox if there are 10 consecutive failed passcode attempts.
To change your passcode, just go to the Passcode Lock settings again.