want to cancel monthly but with a 1-year contract what to do without having to pay arrears.

make sure you're aware of the cancellation terms by selecting your plan type (at the top of the page here), https://helpx.adobe.com/manage-account/using/creative-cloud-subscription-terms.html .  if you have a reason for why you should not be obligated to fullfill your annual subscription, contact adobe support and make your case.

for info on how to cancel your subscription, this page describes the few steps involved, https://helpx.adobe.com/manage-account/using/cancel-subscription.html .

if you want to cancel but are unable to follow the steps described in the "how to cancel page", contact adobe support.

there are 3 ways to contact adobe; chat, phone and twitter:

chat:
use a browser that allows popups and cookies, and click here, https://helpx.adobe.com/contact.html?rghtup=autoOpen
in the chat field (lower right), type AGENT
be patient, it can take quite a while to reach a human.

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

twitter:
tweet @AdobeCare

p.s. if you're contacted by anyone (via email or private message), it's much more likely to be a scammer than an adobe representative. ie, double check for an employee badge if contacted in the forums and look for an adobe.com domain in the email address if you click reply to an email. then check again and be very suspicious. any other method of contacting (or offering to contact you) is almost certainly a scam, https://community.adobe.com/t5/using-the-community-discussions/staying-safe-in-the-adobe-support-community/td-p/12919684/redirect_from_archived_page/true

If you cancel a yearly contact early, there is a fee. It is 50% of the amount left to pay in the current year. To cancel with no fee, cancel in the 12th month. But not a day late, because it starts a new year. It is always cheaper to pay the fee than to wait, though. 

Not sure you can but you could try Adobe support.