C# Sqlite | Can I INSERT 2 times for 1 row?

If I do this

string LoginDTstr = "INSERT INTO Login_Details (userName) VALUES('" + userName + "');";

and I would like to INSERT again on old row. How should I do? I already try on

cmdCount.CommandText = "SELECT COUNT(*) FROM Login_Details;";
long countRow = (long)cmdCount.ExecuteScalar();
string LoginDTstr = "INSERT INTO Login_Details (password) VALUES('" + password+ "') WHERE rowid = "+ countRow +";";

Please Help me. Thanks all.