Open up the category you wish to change the image on.
Go to the linked items tab
Click on the orange ADD button
Select Hero Image
A box opens on the right on your screen with all the images you have on your site. you can either..
Double click on the image you want as the image OR
Drag an image off your computer into the Upload Images box at the top OR
Press the green circle with the white cross and select an image off your computer this way.
The image you have selected will now appear in the Linked items page.
To turn off the old image find it on the linked items page and double click on the white icon to the left of the image that looks like a mountain with a sun over it. This turns off the old image for you.
Go the the live site and refresh and check the image