Change a local windows user password with administrator

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C#

directory

I have a windows server 2012 R2 which not in AD Server.
I have a user "TestUser" on server which can not change its own password.
So, I need login as administrator to reset "TestUser" password.
I tried the codes below, but got access denied error.
How can I change "TestUser" password?

What I have tried:

DirectoryEntry directoryEntry = new DirectoryEntry(string.Format("WinNT://{0}/{1},user", computerName, "Administrator"), "Administrator", "AdminPW", AuthenticationTypes.Secure);

foreach (DirectoryEntry childEntry in directoryEntry.Parent.Children)
                {
                    if (childEntry.SchemaClassName == "User" && childEntry.Name == "TestUser")
                    {
                        childEntry.Invoke("SetPassword", new object[] { newPassword });
                        childEntry.CommitChanges();
                    }
                }