1. Click Products on the navigation bar and select PriceBooks from the drop-down
2. Select the PriceBook for which the product price or discount has to be changed
3. You can view the list of products along with price, discount and tax below the Products tab. Just change the price or discount of the corresponding product as per your wish and click on Save All to save the changes made
NOTE: Expired PriceBook cannot be edited or enabled again
NOTE : Refresh the Register / Billing Screen to update the changes made
Refer the following article to know How to add a product into the existing Pricebook?