Template customization: sidebar and archive title

Hi,

1. I want to remove the word “Category Archives:” on the display page of post category.
Where can I find the file to change that?

https://www.dropbox.com/s/ltoqhtpx4insip2/Screenshot%202016-07-08%2000.08.58.JPG?dl=0

2. I have managed to move the sidebar to the left on normal view by changing the #primary and #secondary floats.
But when seeing at smaller screen (where there is only one column) the sidebar (#secondary) is still at the bottom (after) the main content (#primary).
What I want is to put the sidebar on top of the main content.
What css should I change to do this? or what file is the code in?

Thank you