To begin, locate the item you are looking to discount within the Line Item Group section of your project. Scroll down, find the item, and select the three-dotted edit symbol. Select Edit from the drop down menu.
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A pop-up menu will appear for you to edit the item. You can just navigate to the Discount box. Here, you can make the change by percentage or dollar amount. Enter the discount and press Save. 

The discount will be applied in your project inventory. 

Note: this discount will NOT be displayed in your actual inventory. It is a one-time only discount.