Would like to add, I use the Home inventory template file for my items. In order to update handling time, it required me to fill out the quantity column as well.
When uploading the update file, Step 1 allows you to check the file for errors prior to doing an actual upload. The errors I got were:
Type of error Error message example Affected Attribute Names (Excel Column)
99006 A value is required in the “Quantity” field because there are values in dependent fields.: [Handling Time = “2”]. Quantity ®, Restock Date (W), Handling Time (X)
It mentions Restock Date in the error, but I just left it blank. I copied over the quantity column (that data is included in the inventory report which you download to get your list of SKUs), rechecked the file, got no errors, did the actual upload (Step 2), and all went well. All my items have 2 days handling time now.