This tutorial assumes you are already logged in to your Joomla admin panel
1) Click Site
 
2) Then User Manager
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3) Check the box next to the administrator

Scroll right
4) Click Edit
 
5) Enter a new name in the Username field

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You can also change the password
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6) Click Save
 
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That's it! You now know how to change the admin username and password in Joomla