What if I forgot my passcode?
You can reset your passcode by uninstalling and reinstalling the Dropbox app on your phone. Note that uninstalling the app will remove any files you've marked to be available for offline access.
If you have trouble with the steps above, you can also sign out of your device remotely on dropbox.com.
The next time you open the Dropbox app on your device you will be prompted to sign in, but a passcode won't be required.