Changing an Apple ID from one @icloud.com address to another
From the Apple Support page about changing your Apple ID (emphasis added by me):


  If your Apple ID email address ends with @icloud.com, @me.com, or
  @mac.com, you can only change your email address to another
  @icloud.com, @me.com, or @mac.com email that's already associated with
  your account.


https://support.apple.com/en-us/HT202667

If you created your Apple ID recently with an @icloud.com address as identifier, you most probably can't change that. I think the support document I linked to is targeted at users who have old @me.com or @mac.com addresses that Apple used to issue a long while ago. Or maybe there could be special situations where you actually have two @icloud.com addresses associated with one Apple ID as the result of merging two IDs with the help of Apple Support, but those would be rare exceptional cases. For a recently created @icloud.com style Apple ID, typically you can't change it.