If you have bought a Dropbox from your iPhone, iPad, or other OS devices, you can cancel your Dropbox subscription or free trial by following these steps:

• On your iPhone or iPad, tap Settings.
• Tap your name.
• Tap iTunes & App Store.
• Tap your Apple ID at the top of the screen, then tap View Apple ID.
• Tap Subscriptions (about midway through the screen).
• Tap Dropbox.
• Tap Cancel Subscription or Cancel Trial.
• Tap Confirm.

Your account will downgrade to Dropbox Basic (free) at the end of your billing cycle.