You cannot change your Gmail address after the account is created
Create a new Gmail account with the email address you want
Add your old Gmail account to the new one for email forwarding if needed
Set up automatic forwarding from the old Gmail account to the new one
Update your email address on all important accounts and services
Export your contacts, emails, and data from the old account if needed
Inform your contacts about your new Gmail address
Use the new Gmail address for future sign-ins and communications