It is not possible to update a user id once it has been created.
If the user id was set up incorrectly or a user has changed names we have two options.
1. Leave the user id as it is and update the other info to the new info.
This is not recommended and can be very confusing
2. Delete the original user id and set the user back up with the correct info
Be sure to remove the user from any approval security groups or matrices before you delete the user.