First, I would never put an ASK field in a header or footer. I would not want it triggered every time I print.

If you know where in the document your ASK field is located, you can select that area even with the field codes hidden and press F9 to update. You can display field codes to find it with Alt-F9. Be sure to toggle back to hidden, though, or the codes will print instead of the result.

If you don't have other fields where updating would be a problem, and if your ASK field is in the body of your document, you can simply press Ctrl-A to select the entire body of the document and then F9 to update fields.