Hi Paul,
To install a Joomla component, you would not use php files, but use an intact zip-file.
I sent you the download link to the ExtPlorer. That was a zip file, leave the zip intact and do not unpack.
To install/uninstall you would go to Joomla > Extensions >Install/Uninstall and then choose the ExtPlorer zip file you downloaded earlier.
Once ExtPlorer is installed, you will have a new component called ExtPlorer. When you go into that ExtPlorer component, you will have a file browser similar to the file browser in your computer. Only you are browsing files on your web server.
To upload and overwrite the logo, you would go to <joomla root folder>/templates/rt_versatility_j15/images/style<yournumber>/
Then you can upload files from your computer to the webserver by using the arrow up. Click the arrow up, browse your local compuer for your logo - which must be named logo.png - and upload. It should overwrite the existing logo.png If al went right, you can now refresh your page on your website and should see your new logo. Remember. Logo needs to be in PNG format and best results you will get when you have made the same size.