You cannot change the part before the @ for an icloud.com email address it is the unique identifier for the email address and is forever attached to it.
You can create a completely new iCloud account if you want and use that to sign in to your devices, but all content and data from the previous account will remain tied to it.
click here ➜ How to create a new Apple ID - Apple Support
You can however leave Family Sharing by following the steps in the support document linked below.
click here ➜ Leave Family Sharing - Apple Support