You cannot change the Gmail address of an existing account
You can change the name shown on outgoing emails in Gmail settings
You can create a new Gmail address if you want a different email address
Sign in to Gmail
Click the gear icon and select See all settings
Open the Accounts and Import tab
Under Send mail as, click edit info to change the display name
Save changes
If you want a different Gmail address, create a new Google account with the desired address
Update your contacts and services with the new email address
Optionally set up forwarding from the old Gmail account to the new one