Thank you for posting to this public discussion forum, @muratdanaci.
I appreciate your interest in our services. I see that your new discounted subscription is new. To redeem your new code, you will need to cancel your current commitment.
We don’t offer any student subscription plans for Creative Cloud with ASUS monitors, so you would start a full-price subscription in addition to your discounted subscription if you redeem the code without canceling your current plan as suggested by @kglad.
You can use the steps listed https://helpx.adobe.com/account/individual/subscriptions-and-plans/renewals-and-cancellations/cancel-adobe-subscription.html to self-cancel your current subscription. As mentioned by @John T Smith you will also need to requalify for the discounted plan to start a new discounted subscription.
To redeem the code you received with your ASUS monitor, @muratdanaci , then see https://helpx.adobe.com/download-install/apps/licensing-activation/serial-number-redemption-code/redeem-adobe-code-gift-card.html. I don’t know what type of offer you have received, but the typical offer is for 3 free months followed by the remaining months at the regular price.
As suggested by @kglad you will need to be careful with applying or using redemption codes. I don’t know if it will be worth the 3 months, @muratdanaci, since you already have a full Creative Cloud subscription at a deeply discounted rate.
If you are truly looking to save money, check with your educational institution to see if they offer their own plan you can subscribe to, @muratdanaci. If they do offer their own plan, then your individual plan may not be compatible while using their network or devices, or when interacting with other members of the school.