You can mass edit listings placed on a particular site via the special interface.
To find it, click on the icon of the active site where you'd like to mass edit listings or fix errors.
You'll be redirected to this site's management interface. Here you're able to mass-edit all the listings created on this site; to choose the right categories, set prices or allow paid insertion options if you're out of free listings.
How to mass edit categories?
Open the Filters panel and select the filter Categories > Not specified. Then click "Apply".
You will see the full list of items for which it wasn't possible to specify a category automatically. You can tick all of them if they have the same category or use a filter by Title to find a group of similar items.
In the Category column, start typing the category name or ID
Select the most relevant one and click "Apply changes". Application of changes can take up to 10 minutes.
The publication status, price for a particular site and paid insertion options can be mass-edited in the same way via the mass editing panel.