You can reset your passcode and turn off the passcode prompt by removing your Dropbox profile:
This will remove your Dropbox profile credentials, but it will not affect the contents of your Dropbox folder. The next time you open the Dropbox app, you will need to sign in using your password, but a passcode will not be required.
If that doesn’t work, you can also sign out of your device remotely from 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.