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();
}
}