For your existing listings, you can change Item Specifics, if needed. There are two ways to do it.
1. You can change Item Specifics for the whole category, all you need to do is go to
Sell on eBay -> Configuration -> Categories In Use; select the category you require and apply "edit" action.
with this option, all changes you have made will be automatically applied to all M2E listings where this category is used.
2. Also, you can modify Item Specifics for each product individually. In this case you need to go to the actual listing, change "View mode" to "Settings" ( check the article) and apply "Edit eBay Categories" action for each product.