By the default of wordpress, you can’t change the username of admin when you already installed the wordpress.
Here the simple way to change the username of admin,
Go to Plugins > Add New
Enter the “Username Changer” Plugin name and search it. You can get the plugin first just install and active it.
After activating the plugin you have to move to Users > All Users
Edit the admin username which is in the list where the role is “Administrator”.,
When you click the change username link, the input box of the username will enable automatically and we will change the username as we want whatever after that save the profile of the users.