Where do I write to cancel my trial subscription?

As Axek mentioned here, you will need to contact Adobe customer service, specific to your country where you purchased the subscription.

If you're on a standard yearly subscriptions, you won't get any refund for remaining months as it's a yearly commitent (and lower price).

Please take a look here: How to cancel your Adobe Creative Cloud plan or membership 

If this doesn't help you have to contact the Adobe support. This is a public forum, not the link to Adobe support. Sorry, but we're normal users  and we can't help with account and/or payment problems.

Please click the link below to contact Adobe staff to help.

Be sure to remain signed in with your Adobe ID before accessing the link below.

You have to allow 'cookies' in your web browser for this to work

https://helpx.adobe.com/contact.html

Click the chat icon in the lower right and type "agent" to get a human and not the chat bot.

If you like you can also use Twitter (now X):  Adobe Care (@AdobeCare) / X

Also - beware of answering anyone who sends you a private message or an email.

https://community.adobe.com/t5/community-help/avoid-phishing-3-easy-ways-to-identify-adobe-staff/td-p/10791730