If you made your reservation directly via the CitizenM website, you may use my booking page to cancel your reservation at any time. You may or may not be charged depending on the kind of rate that you selected for this transaction.
If you made your reservation via a third party (such as another website, your travel agency, or anybody else), you will need to contact that party directly in order to cancel your reservation. It is probable that their contact information will be included in the email that confirms your reservation. At the time of booking, the kind of pricing that you choose will determine whether or not you will be charged.