How to add/edit quantity rules and volume pricing

Login as an admin on shopify

On the left pane, go click on "Products" then "Catalogs"

Select a catalog to add/edit

In the next screen click on the 3 dots to see options. Select "Manage products and pricing"

Use the search function to find specific products to edit. Depending on which actions you want to do, click on  "Add" button under Quantity Rules or Volume Pricing

Specify the quantity according to the rules you want to apply on a product when purchased.

Click on "Add quantity price breaks" to add price breaks.

Add price breaks as many as you want then click on Done.