You can change plans anytime under Billing → Plan. Upgrades take effect immediately and are prorated, so you only pay the difference for the remainder of the current cycle. Your new limits (participants, cloud storage, meeting length) apply at once.
Downgrades take effect at the end of your current billing period so you keep what you paid for. If a downgrade would put you under a usage limit — for example, more recordings than the lower plan allows — Orbit warns you before confirming.