To update a user's details click onto the user page. From here click on the drop down arrow of the required user:

We don't allow you to edit a user's details if they are involved with another venue. As this person's user-account specifics (name, contact number and organisation) are shared between all venues with which they are involved, they are the only one that can change these details.
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The only item that you can't edit at all is the email address. For security reasons we don't let you edit this, only the user may edit their own email address. If you have added a user with the wrong email address, a simple remedy is to delete the incorrect user and re-add the user with the correct email address.