When looking at your screen of the POS (see below), scan the product.

Press on the product so that a option screen appears.

Next select Price Override and put the new price. After you put the price press Override Price.

It will ask you Change Product Permanently in Pricebook? If you select yes then it will change permanently but if you select no then it will only change for that transaction.

Once you select the option you want, the price will show the difference on the screen.