In order to remove mSpy from the target device please go to Settings -> Security -> Device administrators:
(for Samsung: Settings -> Lock screen & Security -> Other security settings -> Device administrators - disable 'Update service'.)
(for Huawei: Settings -> Advanced settings -> Security -> Device Administrators - > disable/turn OFF 'Update Service')
(for Xiaomi: Settings -> Additional settings -> Privacy -> Device Administrators - > disable/turn OFF 'Update Service'
Go back to Settings -> Apps/Application manager -> find and uninstall 'Update service'.
The mSpy app is now uninstalled on the target phone. Now login to your mSpy account -> go to the tab called Device management (the last tab in the left-hand side list) and hit the red button 'Re-install'. This button will unlink the target phone from your mSpy account.