By creating a new account and pulling over your content
While you can't change your gmail email address, you can transfer much of the content manually to a new address.It is not possible to change your username in gmail. The username is chosen at the time the account is created and is the permanent identifier of the account.
But if you need to use a different username, it is possible. You simply need to create a new account and move your content over from the old account. Here's how.
Step 1: Create a new account
You should sign out from your existing account and then choose Create an account from the gmail login screen.
Step 2: Grab your mail from the old account
Login to the new account you just created, go to the settings link in the top right, and choose the Accounts and Import tab. Gmail doesn't let you use its standard import feature to transfer from one account to another, so you'll need to use the Check mail using POP3 entry. Enter the following configuration:
Username: Your old gmail email address
Password: Password for your old gmail account
POP server: pop.gmail.com
Always use secure connection: checked
Label incoming messages: checked
This will pull over all your old mail and will continue to pull over new mail received at the old account until you disable it. It will not, however, save any label information. You will need to relabel your emails (or simply rely on gmail's search features to find messages).
If you absolutely need the old label structure, the only solution is to use an IMAP client to connect to both the old and new account and copy over folders/labels. But this solution can be tricky, because you can only copy over a certain (unspecified) amount of mail at a time. The non-Google program Gmail Backup might be able to assist with this process, but I can't vouch for its effectiveness.
Step 3: Export and import your contacts
Login to the old account and follow the instructions on the Export Gmail contacts page. Remember the location of the saved file.
Login to the new account and follow the instructions on the Importing CSV files page using the file you just saved.
Step 4: Export and import your filters
If you have many filters on the old account, you can use the Filter import/export feature of gmail labs to bring them to your new account.
Step 5: Non-Gmail Google account contents
If you have other content in your Google account that you want to transfer (calendars, documents, photos, etc), you'll have to do this separately. The Data Liberation Front website has detailed information on how to export and import data from pretty-much every Google product.
You should carefully examine the settings on your new account to see if there is anything you need to replicate from the old account.
It is best to keep the old account around so that you can keep receiving mail to the old address. You can consider activating the vacation responder to alert people who send to the old address that your address has changed. But if you really need to delete the old account, you can follow the instructions on the Canceling your Gmail address page, while logged into your old account. Remember that if you delete an account, you can never get it back or reuse the address.
Original created by Joshua S on Knol.