Came across this problem while testing some workflow approvals on my SharePoint 2013 instance, needed to change the email addresses of my test users.
Run this powershell script to programmatically update the email address of the SPUser object.
$web = Get-SPWeb "http://sp13dev01:8081" ;
$user = $web.EnsureUser("empowercs\man1");
Set-SPUser -Identity $user -Email "firstname.lastname@example.org" -Web "http://sp13dev01:8081"