Upgrading, downgrading and cancelling all happen on your billing page. There is no annual contract to unwind and nobody to email first.
Pick the plan you want and pay through Stripe. The higher lead limit applies straight away, including to any leads that were waiting over your old limit.
Cancel from the same page and your plan runs to the end of the period you have already paid for, then drops back to Trial. Your leads, threads and settings all stay exactly where they are — you are not deleted, you are just back on 25 leads a month.
If you want your data removed rather than just your plan cancelled, see 'Your data and how to delete it'.
Manage your plan