If you have an attribute field that is string type and it needs to be numeric (e.g., parcel land values are numbers but in string format, and you want them to be true numbers) you can do the following:

1. Open the attribute table and start editing - choose Table-Start Editing
2.  Create a new attribute field (Edit - Add Field) that is numeric in type. Make sure that it is wide enough to include your largest value, and that it has decimal points if needed. Click OK.
3. The new field name should be highlighted in the table (if not make sure to highlight it). Choose Field-Calculate or press the calculator icon ()
4. For the expression, double-click on the original field you want to change so that it shows up in the expression (e.g., [land_value]), then type the following:
   .AsNumber
5. Your entire expression should look like this:
   [land_value].AsNumber
6. Assuming this works and your new field is filled, stop editing and save your edits.

To go from a numeric value to a string value, you would follow the same procedure but use the expression .AsString

Note: The X-Tools extension has a function under the Table X-Tools menu called Table Structure - this will show you the structure of all the attribute fields in a table, including what type they are.