How to set password for user in Window Authentication

using System.Security.Principal;
using System.DirectoryServices;

        private Boolean AuthenticateWindowsID(String username)
        {
            WindowsIdentity ident = WindowsIdentity.GetCurrent();
            WindowsPrincipal user = new WindowsPrincipal(ident);

            using (DirectoryEntry de = new DirectoryEntry("LDAP://" + StripDomain(user.Identity.Name)))
            {
                using (DirectorySearcher adSearch = new DirectorySearcher(de))
                {
                    adSearch.Filter = "(FirmAccountName=" + username + ")";
                    SearchResult adSearchResult = adSearch.FindOne();
                    if (adSearchResult != null)
                        return true;
                }
            }
            return false;
        }