You can quite easilly transfer and use existing licence(s) on new devices.
Download CCleaner Free onto your new device and simply activate it as Pro using your existing key. It’s that simple.
You should of course uninstall CCleaner Pro from the old device, but you can then put CCleaner Free on that without activating it.
CCleaner realises that people do change devices and so will allow short term use of CCleaner Pro keys on more devices than you are licenced for.
(however if someone tries doing it long term then that licence will be cancelled for breach of the terms).
You can find an installer for CCleaner Windows here: Download CCleaner builds | CCleaner Portable, Slim, and more
If you do want to cancel and get a refund that you would need to contact support for that.
However we see some stories that it’s not always easy to get a refund, so if you are happy to transfer the licences to your new kit then that will be less stressful.
If you do want to contact support then you can raise a support ticket by using the form here: CCleaner Support Community
(Note do not try to change the language of that form, it has a bug and becomes unresponsive if you do that. If it happens then close and reopen your browser and try the form again without changing the language).
However you should note that support are slow to answer these days and it can take a week or more for a registered customer to get a reply from a support agent.