External ID of an item record is a standard field that cannot be modified through user interface. Modification can only be accomplished through CSV Import.
1. Create a CSV file, basic example is to add 2 columns:
a. Internal ID - internal ID of the item record to be updated. This can be located on the item record page under Store tab, or just by viewing the ID parameter value of the item record's URL.
b. External ID - enter value that needs to be added as item's external ID
2. Navigate to Setup > Import/Export > Import CSV Records
3. On Step 1 of the Import Assistant Page:
- Set Import Type = Items
- Record Type = Inventory Item (or which ever item record that needs to be updated)
- Upload CSV file
- Click Next
4. On Step 2, select Update then click Next
5. On Step 4 Field Mapping, make sure the Internal & External fields are mapped appropriately to the corresponding NetSuite fields. Click Next
6. Step 5, continue to save & start importing
To check if the external ID was updated, user can create an item saved search and display external id on the search result to see if item's id was successfully updated.