Changing Appearance of Category Pages


I can't figure out how to change the look of my category pages. In fact, when I try, it applies the changes to a page that I don't want to change at all.

When I go here, for example:

You can see that it is obviously an Upfront placeholder.

But when I try to edit it in Upfront, on the left it says the template is "undefined". When I make changes, Upfront crashes, and when I reload to see if the changes took hold, it turns out they got applied to this page: is what I want the Category pages to look like but I can only seem to figure out how to do it all backwards.

Please help me change the Category pages so they look exactly the same as my main blog page here: