You need to use an FTP client, like FileZilla. Your web host should have provided you with an FTP user ID & password to your account.
Once you connect to your site via FileZilla, the left pane will show the contents of your local computer while the right pane will show the contents of your web site, sort of like Windows Explorer. Navigate to the /wp-content/themes/atahualpa/images/header folder on the right pane and you'll see the images that are used for the header logo. Drag & drop any images from the left pane over to the right pane that you want to use for your header.
If you are just starting out building a site, is there a reason why you decided on Atahualpa instead of Montezuma? Both were developed by BytesForAll, but Montezuma has the advantage of being responsive (i.e., adapting to mobile devices). Not that Atahualpa isn't an excellent theme, I've used quite a bit myself, but I've been converting my Atahualpa sites to Montezuma so they're mobile-friendly.