First of all, be aware that your account is shared between the Play Magnus and Magnus Trainer apps. If you delete your account, all your data and progress in both apps will be deleted. Also, know that once your data is deleted we will not be able to restore it.
One option is to go into More, then SETTINGS, and tap on Delete User.
Another option is to go to https://www.playmagnus.com/en/account and click "Delete account"
If that for some reason doesn't work, you will need to send us an email at firstname.lastname@example.org from the same email account as you have registered in Play Magnus. Please also include your username in Play Magnus so we confirm your identity. In this case, please let us know that you couldn't delete your user from the app.