On your shopping list screen, swipe left on the shopping list row that you want to edit as shown below and tap on the edit icon.
Edit a list by swiping left on a shopping list row and tap on "Pencil" icon to open the Edit screen. Now go to the bottom of the screen and enter a default tax rate for the selected shopping list. Once saved, the entered default tax rate will automatically apply to all items in the shopping list. Note that an item can override this default tax rate by setting its own tax rate.