In the library, make your listing selections and click the Bulk Actions button.
The Bulk Actions button will appear towards the top right of your screen once at least one listing is selected.
Choose Edit Listings, then Handling Time.
In the popup, select to Edit this page of listings only or Edit all listings at once and select Next.
If you selected to edit this page of listings only, on the next page you're able to change all of the listings at once or set a handling time for each individual listing. Please note that if you're using shipping profiles, handling time should be updated within your profile and not bulk edited in the library.
Once you've completed the changes, select Save all modified.
If you've chosen to Edit all listings at once, set your value in the popup and select Edit Listings.