Click on 'Manage Inventory' under the 'Inventory' heading in the Control Panel menu. 

Use 'Find by ID' or 'Keyword Search' textbox to find the Item. (Or you can use 'filter' icon next to keyword search textbox for 'Advanced Search')

Click the 'Edit' icon at the end of your item to go to it's settings page.

On settings page, look for the 'Discount' dropdown menu and keep the toggle switch on for 'Allow Discount' label. 


Specify the amount for 'Discount' using the dropdown menu. Then, click on the 'Save' button.

To remove the discount, return to this screen and change the discount amount back to 'None' from the 'Discount' dropdown menu.