From the help of Access VBA
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
I would open the table and change the field type by selecting the Text data type!(save the change)
If it is VBA then...
CurrentDB.Execute "ALTER TABLE tblName ALTER COLUMN YourField CHAR"
or (MS-Access 2000+)
CurrentProject.Connection.Execute "ALTER TABLE tblName ALTER COLUMN YourField CHAR"