Yes, at any time and from inside the platform.
Yes. Plan changes are made in Settings, under Subscription and Billing, and take effect immediately.
Upgrading charges the difference for the remainder of the current period, prorated, so you pay only for the time you actually get. Downgrading does not produce a refund: the value you have already paid for is converted into additional time on the lower plan, so a downgrade extends your subscription rather than returning money.
Cancelling stops the renewal. You keep access until the end of the period you have already paid for, and your data remains available afterwards under the Starter feature set.