From GarageSaleWiki

If you need to modify a listing after you started it, you can revise your listing inside GarageSale.

Watch the tutorial video:
Revising Auctions

To revise a listing, switch to GarageSale's template mode or in GargeSale 4 and newer use the Inspector and...

1. Select the auction template you started your listing from. 

2. Make the changes you want to incorporate into your listing to the selected template. 

When you are done, go to the templates 'Auction' section (GarageSale 3 only) by clicking the switch control at the top right, (NOT the mode buttons in the top middle!).
In GarageSale 4 and newer, you simply have to use the Inspector (tab Auctions).

3. From the auctions table, select the auction to revise and click the revise button.

4. Now choose what parts of the auction to revise or select 'Revise entire auction'.  

In case you brought up the revise panel from the menu item in the Template menu while having several auction templates selected, you can now choose what auctions in you want to revise exactly in the upper part of the window. Note that you can either select an auction template, which will include all auction started from that template in your revise, or cherry-pick individual auctions.

5. GarageSale will then revise your auction...

Note, there might be several reasons why eBay might not accept your modifications!!!
In this case, of course, GarageSale will warn you:

Revising multiple auctions

In GarageSale you can also revise multiple auctions in batch. To do so select the auction templates you want to revise and from the Template menu choose Revise Running Auctions....