Upgrading, downgrading, switching from monthly to annual and cancelling are all done from the dashboard, without ringing anyone or writing an email. There's no minimum term.
Changing plan
The change happens there and then, and so do the sums: on an upgrade you're charged the difference for what's left of the period and the new credits are available straight away; on a downgrade you're left in credit, and it comes off the next invoice. Waiting for renewal would leave someone who has just paid more without what they asked for until the following month.
Cancelling
The cancellation is scheduled for the end of the period: you keep the plan until what you've already paid for runs out, and until then you can undo it with one click. Changing plan undoes it too, because changing plan means you want to carry on.
When it ends, the account moves to the Explora plan and nothing is deleted: the profile, the saved calls, the applications, the tasks and the Library stay where they were. So do the credits bought in a top-up, because they don't expire.
The team seats
The number of seats doesn't go down mid-period: if you take someone off, what was already invoiced keeps being charged until renewal, and there the count goes back to the people actually on the team. Inviting does put it up straight away, and you're charged for the part of the period that's left.
Cancelling the plan isn't deleting the account: it leaves it on Explora with everything inside. If what you want is for it to disappear, see deleting your account.