On dropbox.com, click your avatar, choose Manage account, then Cancel plan at the bottom of the page. Your account moves to free Dropbox Basic at the end of the billing cycle.
Steps
On the website
Log in at dropbox.com.
Click your avatar in the bottom-left corner.
Click Manage account.
Click Cancel plan at the bottom of the page.
Select a reason, click Continue canceling or Confirm change, and follow the prompts.
Billed by Apple or Google?
If you subscribed inside an iPhone or Android app, the app store bills you and you cancel there.
Billed by Apple (iPhone or iPad)
Open Settings and tap your name.
Tap Subscriptions, then the subscription.
Tap Cancel Subscription.
Billed by Google Play (Android)
Open play.google.com/store/account/subscriptions on your phone.
Select the subscription.
Tap Cancel subscription and follow the steps.
When it ends
At the end of your current billing cycle your account moves to Dropbox Basic and you aren’t charged again. Your account isn’t deleted.
Good to know
Basic includes 2 GB. If you’re over that, you may not be able to sync, upload or share files, and Dropbox may delete files you own to get back under the limit. Download or delete files first.
No Cancel plan button? You probably bought the plan on a phone or tablet. Cancel it in that device’s app store (steps below).
Look for an email from Dropbox with the subject “Dropbox Plan will not renew”, or a “Plan downgrade scheduled” notice on the Billing tab.