Commerce Product Collection View Block: What Determines CSS For Each Header Link?
It is the same as with any view: You have to override the view template. 

In your views UI click on 'Theme:information'. This will open up a dialog with views templates and suggestions. As the block is using grouping, you need to override the 'Style output' theme template: 

views-view-list.tpl.php


If you override that template it will override it for all views of HTML List format, so you now have to select a suggestion. Second in the list is:

views-view-list--collection-products.tpl.php


which will be fine.



Create a new file in your theme templates folder called

views-view-list--collection-products.tpl.php


Then click on the 'Style output' link back in the view theming information. It will open up a file and you copy everything from the 



Now you can edit that file to get the markup you want. You can take out the h3 tag for example and replace with a div.