Bulk actions let you specify markups, discounts, and other attributes to select line items.
Start by ticking the ☑️ checkbox next to each line item you want to change, and you'll notice the cog on the toolbar become available.
Click the ⚙ cog and select the action you want to apply:
You can select all items in a section by ticking the checkbox below the section title.
See this article for information on combining to one line.