How?
Click Inventory
Select SKU that you want to export. Ensure that you tick the box beside the SKU before exporting. In case you did not tick the box and proceed on exporting, all product would be exported.
Select Export in Top Right Corner.
Select your desired format.
Note: If you want to export with Custom field, ensure that you tick the "Custom field" column on right side panel.
Once you selected the format, it will automatically download file on your computer.

