Once registered, domain names cannot be changed. Upon purchase, the domain is instantaneously registered to you at the registry for a full calendar year/billing cycle.
You would need to register the new domain you want, and set the existing one to expire in your dashboard if you no longer want it.
If the domain is very newly registered, we may be able to request deletion and refund by the registry. This is only available for gTLDs, not ccTLDs. Please get in touch and we can advise further.