You cannot change the email address of an existing iCloud Mail account
You can create a new iCloud email alias in iCloud Mail
On iPhone or iPad, go to Settings
Tap your name
Tap iCloud
Tap iCloud Mail
Tap iCloud Mail again if needed
Tap Aliases
Tap Add an Alias
Enter the new alias details
Tap Done
On Mac, open iCloud Mail in a browser
Go to Mail settings
Select Aliases
Choose Add an Alias
Enter the alias details
Save the changes
If you want a different primary email, create a new Apple Account with the desired email
Sign out of the old Apple Account
Sign in with the new Apple Account
Update your email address on all services that use the old iCloud email