How do I write to specific cells in Excel? ... C#, OleDb

SET is not used in an INSERT statement. Try this:

string sql = "Insert into [" + stSheetName + "$A2:B2] VALUES('5','e')";

You could also use an UPDATE statement to update a specific cell:

            string sql = "UPDATE [" + stSheetName + "$A2:A2] SET F1=5";
            myCommand.CommandText = sql;
            myCommand.ExecuteNonQuery();

            sql = "UPDATE [" + stSheetName + "$B2:B2] SET F1=e";
            myCommand.CommandText = sql;
            myCommand.ExecuteNonQuery();

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