You cannot change your existing Gmail address
You can create a new Gmail address
Sign in to your Google Account
Go to Gmail settings if you want to change your display name
Click See all settings
Open the Accounts and Import tab
Under Send mail as, click edit info to change the sender name
If you want a different email address, create a new Gmail account
Set up forwarding from your old Gmail account to your new one
Update your email address on important accounts and services
Export your data if needed before switching accounts