You'll have to contact 48 support and ask them if they can facilitate this.
If they can't then the only way to change number is to get a brand new SIM and activate it as a new user/account/email address etc.
To contact 48 support see the link in my signature below or email them at email@example.com
While you're here, check out our "Featured Topics" below. Our Troubleshooting Guide and Knowledgebase contain help and support articles and popular questions to help you as a 48 customer.