Using VirtueMart media import you cannot update file_url field as that is the needed field. You can use Custom table export template to export virtuemart_media_id and file_url field from #___virtuemart_medias table. Edit the file_url field value and using Custom table import template import the updated values to #__virtuemart_medias table. That should do the update you need.
When updating/importing the file_url (aka Used url) do I need the product_sku?
file_url is for importing images. If you looking at updating/importing only images you can use VirtueMart Media import. You don't need product_sku in this import. But I don't understand what you mean by Used URL, May I know what that means?