You can edit your for sale items at any time by going to the item page and clicking on 'edit item' on the right hand side.
Alternatively, select 'Your Shop' > 'Dashboard' from the menu at the top of the page, then click on 'listings' on the left hand side to view a list of your items. Click on 'edit' alongside the item you wish to edit.
Need to add options or variations to your listings? You can do this after you have listed your base item.
Go here to learn how: https://blog.folksy.com/2018/10/16/how-to-use-variations-on-folksy