There are multiple methods for updating inventory amounts for all of products. On the Inventory page, clicking or touching the pencil icon on the far-right lets you edit inventory directly.
If you instead click or touch the name of an item, it will bring up the quick adjustment box. A positive number will add to the stock total, while a negative number will subtract from the item's inventory.
If you would rather adjust inventory of multiple items at once, a menu option at the top allows you to Add or Remove Inventory for all your items at once.
Choose Add or Remove and make adjustments to all active inventory. The button color will update to red if you choose to remove inventory, as a visual reminder.