Hi Devanshi Kacholia,

It's not possible to change or cancel your subscription in advance. So you'll have to wait until a month has passed and then cancel your subscription.

edit: The solution provided by Lorena NS below is much better 

Hi Devanshi, 

After you purchase an agreement, you are entitled to a 14-day cooling-off period. If you change your mind in that time, you can decide to cancel the season ticket and withdraw from the contract without specifying a reason. This is known as your right of withdrawal. You can contact our costumer service to withdrawal your contract. 

Do realize that when you cancel the subscription you will have to pay the full-fare for any trips made in the last 2 weekends. If this is more then 31 euros (price of Weeken-free), the best option is to cancel the subscription exactly after one month.

Unfortunately the NS doesn’t provice the service that you can cancel the subscription in advance (like a normal company).