Most of the user ask for changing the username. In wordpress you can’t change username you can change email and name etc.
When you import new demo, Your database also changes which is why your username and password also change with demo import.
In this case you can create new user. Go to WP > Users > Create new
Role should be administrator. Then signout from Wpsixer user.
And now sign in with your new admin created one.
After signed in Go to wp > User > click delete. Wpsixer user.
two options will be ask
Delete all content.
Attribute all content to:
Select attributes all to : The new admin you have created.