Banner upload is not possible with this template.
In version 4.2 section for banner uploading is grayed out.
Not all templates support upload banner feature. If banner consists of one file then uploading is possible. But for some of the templates banner consists of several files. In this case uploading is not possible. Among existing templates most of them support banner uploading feature.
Banner upload support added for all design templates in latest 4.2.3 version. For more details please reder to this