On the WinPOS (desktop solution): to change the item to tax exempt, double click on the item in the cart. It will bring up the edit item screen, here you can change to tax exempt for the item.
If all items in the cart are tax exempt the option is available on the Sales Transaction screen
On the TabPOS - you are unable to change individual items to tax exempt. You can change all items to tax exempt (from the transaction level options) or if the item should always be tax exempt (non taxable), modify the item to remove the tax from the item.