How to update data in Active Directory using DirectoryEntry?

I want to update user expire date in Active Directory. If already have user then update expire date. I try to call CommitChanges with this code:

if (result == null)
{
    DirectoryEntry newUser = dirEntry.Children.Add("CN=" + fNm, "user");
    newUser.Properties["sAMAccountName"].Value = uNm;
    newUser.Properties["givenName"].Value = fNm;
    newUser.Properties["sn"].Value = lNm;
    newUser.Properties["displayName"].Value = NID_Number;

    dateEng = DateTime.Today.AddDays(3); ;

    newUser.Properties["accountExpires"].Value = dateEng.ToFileTime().ToString();

    newUser.Properties["userPrincipalName"].Add(uNm + "@pkru.ac.th");
    newUser.CommitChanges();
    oGUID = newUser.Guid.ToString();

    const int UF_NORMAL_ACCOUNT = 0x0200;
    const int UF_DONT_EXPIRE_PASSWD = 0x10000;
    newUser.Properties["userAccountControl"].Value = UF_NORMAL_ACCOUNT + UF_DONT_EXPIRE_PASSWD;
    newUser.Invoke("SetPassword", new object[] { NID_Number });

    newUser.CommitChanges();

    dirEntry.Close();
    newUser.Close();
}
else
{
    gp = SearchUserGroup(result);

    if (string.Equals(gp, "ABC"))
    {
        dateEng = DateTime.Today.AddDays(7); ;
        DirectoryEntry newUser = dirEntry.Children.Add("CN=" + fNm, "user");
        newUser.Properties["accountExpires"].Clear();
        newUser.Properties["accountExpires"].Value = dateEng.ToFileTime().ToString();
        newUser.CommitChanges();
    }
}

When I run it show error like this.

at System.DirectoryServices.DirectoryEntry.CommitChanges()
at NIDCardCS.Form1.AddToAD(String fNm, String lNm, String uNm, String) in C:\Users\Test\Form1.cs:line 289
Exception thrown: 'System.DirectoryServices.DirectoryServicesCOMException' in System.DirectoryServices.dll
System.DirectoryServices.DirectoryServicesCOMException (0x80071392): The object already exists.

How to update data in Active Directory using DirectoryEntry ?

If you don't know the path to the user, use DirectorySearcher to find the user. If you do know the path, construct a new instance using. e.g.,

using (var entry = new DirectoryEntry("LDAP://CN=first last,OU=blah,DC=blah"))
{
    entry.Properties["accountExpires"].Value = blah
    entry.CommitChanges()
}

You don't normally need to Clear a value before setting it.

Always use using if you can as it reduces the change of you forgetting to call Close.