Everything is in Settings › Plan.
Upgrading
An upgrade starts straight away. You pay the difference between the two plans' prices for your billing period, from your wallet, a saved card or a new card. Your new plan's price applies from your next renewal.
Downgrading
A downgrade takes effect when your billing period ends. You keep your current plan's features until then, and you can select Keep to undo it.
Cancelling
Your plan stays active until the end of the period you've paid for. After that, it doesn't renew and you won't be matched with new jobs. You can select Resume plan any time before the period ends.