Exactly What he said ^
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Goto 'My Computer' (You can Press Start + E on the Keyboard to pull it up)
Open 'Control Panel' (The Last Text Option on the ribbon with text at the top)
Change 'View by:' from 'Category' to 'Large icons' (look upper right side)
At the end of the list look for "User Accounts" and click on it
Choose 'Change your account name' (3rd option, and first one with a shield to the left of it)