If you go into your account settings, just under your avatar/icon is a
link that says 'change'. Click that, or click on the link that says
'Personal Profile' under the 'My Channel' settings. Those links take
you to a page where you can change your avatar/icon and a bit of other
information.
Note that the change may not be instant, you may have to wait a few
hours for it to show up correctly.
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You can't change your username on YouTube, though you may create a second account with a new name.
Once you create a YouTube username, you will not be able to change it. However, if you were affected by the Google+ to YouTube name change, you should be able to edit your preferences for the name displayed in your account settings.
You cant change it, if you really want to change it the only way to do it is to delete your account and to do that you have to go to manage account, delete account, give a reason, and boom you have deleted your youtube account.
In Avatar: Legends of the arena you just need to click on your name to change it. I am not sure about THAT OTHER webgame sorry.
Avatars,as in icons for MSN,go here - http://99writtenwords.avatardestinyportal.com/avatars.php Avatar the show,as in watch episodes online,go here - http://avatardestinyportal.com Or,avatar as in where can I buy DVDs. Walmart,Amazon.com or Ebay. =) - Jun yea