How to change username for @icloud.com email

You can create a new Apple ID from www.icloud.com , the primary Apple ID credentials will be required in upcoming steps How to create a new Apple ID - Apple Support

Note : you can't change first and last name associated with iCloud domain , however an alias for it can be created .

See the section - Create your Apple ID on the web

Click on cancel .

Fill up the upcoming credentials , click on create Apple ID

To the best of my knowledge, that would require creating an entirely new Apple ID, which is not advised.

You can use aliases.

Add and manage email aliases for iCloud Mail on iCloud.com - Apple Support

Please note the drawback to creating a new Apple ID. Unless you use Family Sharing between the old and new Apple IDs, you will lose access to anything purchased under the original Apple ID. You cannot merge Apple IDs and you can't transfer purchases from one to the other.

Consider carefully before creating a new Apple ID.