Your username and email address associated with your account can be changed only on the musescore.com website, not in the mobile app.
Here is a short instruction on how to change your login details:
Log into your account on the musescore.com website
Go to the Account Settings or click your profile icon located at the upper right of the MuseScore website and choose 'Settings' in the dropdown menu.
Click the Account tab and update the information.
Don’t forget to save changes!
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