To apply the same value to all products at once:

1. In the administration panel, go to Products > Products.
2. Select all products by selecting the left top check box.
3. Click the button with the cogwheel icon in the right corner of the page and select Edit selected.
   

   
   
   

4. In the opened Select fields to edit window, click on the Unselect all link and select the check box corresponding to the necessary option (e.g. Inventory). Click the Modify selected button.
   

   
   
   

5. Click on the Apply values to all selected products link. This will open a new small section.
6. Select the check box corresponding to the necessary field and  enter the necessary text or select the necessary variant in the field  next to the selected one.
7. Click the Apply button to apply the selected value to all products.