I have a monthly subscription. Can I switch to yearly subscription?

Yes. Please sign in to your JetBrains Account, select a domain name in "Your Site" drop-down list, choose a suitable plan and click the "Subscribe" button.

On the Order page, enter all required information, choose your preferred payment method, and proceed to Checkout.

That's all.

An administrator can also change an existing YouTrack Cloud plan from the licensing portal by clicking "Change plan" and selecting the new desired plan on the following checkout page.