how to change type of the cells in excel?

I tried to change type of the column in excel with SQL command: "ALTER TABLE [Customers$] ALTER COLUMN Customer_Name TEXT(30);" Result: Operation is not supported for this type of object. With MS Access table don't have problem, it's works. In the help of the MS Access I found the following text from Microsoft: The Microsoft Jet database engine does not support the use of ALTER TABLE, or any of the data definition language (DDL) statements, with non-Microsoft Jet databases. Use the DAO Create methods instead.
I want to convert all fields in the excel sheet to text type, because I have problem when I try to read trough ODBC cells which contains numbers, and it's type not set to text.

Thanks, Arman.