How to programmatically change Active Directory password

You can use the UserPrincipal class' SetPassword method, provided you have enough privileges, once you've found the correct UserPrincipal object. Use FindByIdentity to look up the principal object in question.

using (var context = new PrincipalContext( ContextType.Domain ))
{
  using (var user = UserPrincipal.FindByIdentity( context, IdentityType.SamAccountName, userName ))
  {
      user.SetPassword( "newpassword" );
      // or
      user.ChangePassword( "oldPassword", "newpassword" );

      user.Save();
  }
}

Here's a great Active Directory programming quick reference:

Howto: (Almost) Everything In Active Directory via C#

See the password reset code near the end.

public void ResetPassword(string userDn, string password)
{
    DirectoryEntry uEntry = new DirectoryEntry(userDn);
    uEntry.Invoke("SetPassword", new object[] { password });
    uEntry.Properties["LockOutTime"].Value = 0; //unlock account

    uEntry.Close();
}

public void ResetPassword(string userName, string Password, string newPassword)
{
    try
    {
        DirectoryEntry directoryEntry = new DirectoryEntry(Path, userName, Password);

        if (directoryEntry != null)
        {
            DirectorySearcher searchEntry = new DirectorySearcher(directoryEntry);
            searchEntry.Filter = "(samaccountname=" + userName + ")";
            SearchResult result = searchEntry.FindOne();
            if (result != null)
            {
                DirectoryEntry userEntry = result.GetDirectoryEntry();
                if (userEntry != null)
                {
                    userEntry.Invoke("SetPassword", new object[] { newPassword });
                    userEntry.Properties["lockouttime"].Value = 0;
                }
            }
        }
    }
    catch (Exception ex)
    {
        Log.Error("Password Can't Change:" + ex.InnerException.Message);
    }
}