You can cancel at any time, with no commitment.
Key points 
Cancellation is done from the billing portal
It takes effect at the end of the period already paid (not immediately)
You keep access to your plan until that date
After that, your account switches back to the free plan
How to cancelFrom the Subscription section, open the billing portal and cancel your subscription. No complicated steps, no commitment.
When it takes effectThe cancellation is scheduled for the end of the current period: you keep access to all the features of your plan until the renewal date, then your account switches to no-subscription mode.
And my galleries?Your content stays on your account, but the free plan limits then apply (3 galleries, 1 GB): remember to retrieve what you need if you exceed these limits.
Is the cancellation immediate? No, it takes effect at the end of the paid period; you keep access until then.
Will I be refunded for the remaining time? No, you keep access until the end of the period already paid.
Can I undo it? Yes, you can reactivate or resume a subscription at any time.
Where exactly do I cancel? In the Subscription section, via the billing portal.