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.
To change your passcode, just go to the Passcode Lock settings again. For even more security, you can turn on an option to erase all Dropbox data from the phone if the wrong passcode is entered over 10 times.