Click the Mail Servers subhead under Mail & Newsgroups to get the
screen below.

Now click the Edit button (if it's not highlighted, click on the Incoming
Mail Server name (mail.turbonet.com) to select it) at the upper right of the
window above.
Change the User Name.
Note that if we host your domain on our mail servers, there may be an
additional "tag" at the end of your username to indicate which
domain this email address belongs to. For instance, Monica@PearlTrans.com
has a username of monica.pt.
Please do not check the box in front of "Check for mail every x
minutes. While this may sound good, it can act as a keep-alive,
keeping your computer connected to the Internet when you've forgotten to
disconnect. This ties up your phone line and it ties up our phone
line, which either costs us extra money or causes busy signals, and we don't
want either.
-- Monica@TurboNet.com