Sorry Linda but we're having trouble understanding what you want to do.
If you are trying to change the data type of the column in the table, open the table in design view and change it. If you have invalid data, it will be lost. A more complicated method is to create a new column to hold the numeric values and to run an append query to populate the column. You can then compare the two columns. Use a condition such as - Not (CDbl(YourOldColumn) = YourNewColumn) to identify the bad records.
If you simply want to convert the field to a double in a recordset, use a query that does the conversion -
Select CDbl(YourOldField) As YourNewField, ....
From ....