The steps differ depending on your payment method.
You will still have access to your plan's benefits until the end of its current validity period, even after cancellation.

For in-app purchases
For website payments

For in-app purchases

You can unsubscribe from Pastela Plus from the Settings app on your iPad.
Open the Settings app and tap your Apple Account username > Subscriptions > Pastela (plan name) > Cancel Subscription.

*Make sure you’re signed in with the Apple Account used for the payment.
If you’re signed in with a different Apple Account, log out and sign in with the account you used for the Pastela Plus payment before proceeding.

*Your plan will remain active until the end of its current validity period. You’ll need to wait until it expires to make the switch.

For website payments

You can unsubscribe to Pastela Plus through the website (Pastela Cloud)’s plans page.
Please switch to the Free Plan from this page.
*When opening the page, make sure to log in with the Pastela (pixiv) account you are using for Pastela Plus.