you can export your products to a csv file (UTF_8), with the field "pos category" and the product "name"; then modify the pos category rows (using any spreadsheet tool) ; once done you import the csv file.

If you want to do it manually, go to "Sales/Products" under the Sales tab (if I remember it correctly) and assign the corresponding POS product category. This is at the same place where you also assign the Website product categories.