Change User Email using Membership

Try this:

Dim user As MembershipUser user = Membership.GetUser(UserName)
user.Email = TxtBxNewEmail.Text
Membership.UpdateUser(user)

-- But check for email availability:

public bool CheckEmail()
{
bool emailExsit = false;
MembershipUserCollection users = Membership.FindUsersByEmail(Email.Text);
foreach (MembershipUser user in users)
{
if (Email.Text == user.Email)
{
emailExsit = true;
break;
}
else
{
emailExsit = false;
}
}
return emailExsit;
}

Good Luck