If you're looking to switch your plan, you can do so through the Dropbox website:
If you're looking to cancel your subscription altogether, the Downgrade to a Free Account link is at the bottom of the page.
If you downgrade, you still get to finish out any remaining time on your current subscription before any new changes take effect.
Downgrading to a smaller Dropbox won't delete the files already there. For example, if the amount of disk space you use is above 18 GB and you downgrade to a Free 18 GB (2 GB + 500 MB per referral) account, the entirety of your Dropbox will still remain. However, you will not be able to add any new files to your Dropbox or return any files removed from your Dropbox until there is enough free space in your storage quota to do so.
If you are switching from a Dropbox Pro account, any remaining balance will be automatically applied to cost of your new plan or prorated toward future payments.
If you're the admin of a Dropbox for Business account and you wish to downgrade, please contact us using the information on your Business account page and we'll take care of this for you. Keep in mind that there is a 30-day refund policy but if your request is made within that time, we're happy to provide a full refund for your purchase.