To begin head over to the Shop tab. Once there find the "Download Listings" segment.
You need to select the type of export you want and which listings you want to export.
Type of Export
Export SKUs and inventory
This only exports listing_id, skus, quantity, action, and listing_state
Export all product information
This exports everything into a template
Listing States to Export
Draft selected
Your export will only contain draft listings
Active selected
Your export will only contain active listings
Both selected
Your export will contain both draft and active listings
Once you are happy with your selection click "Start Export".
After a bit, your export will show up in the "Export History" section. Once that happens click the download button.
I already have a Shop Uploader template
If you already have an existing template then all you need to do is enter your product's listing ids into the listing_id column. You can do this by using vlookup, hlookup, or index+match from the export into your template. Once that is in place then you can start editing your information.
Notes:
To edit only inventory set the action column to "inventory"
To edit everything else set the action column to "update"
To change your listing from draft to publish set the action column to "state_change" and listing_state to "published"
I do not already have a Shop Uploader template
For more information on how to create your Shop Uploader template visit the How do I add product properties to my Shop Uploader templates? article.
Once your template is set up and updated you can upload it to Shop Uploader. Assuming there are no errors it will then update your existing Etsy listings.