Ensure your PC has the Nokia Connectivity Cable Drivers installed so it can recognize the phone via USB. Sources like Driver Scape host these drivers.
You cannot download via the phone itself anymore. The servers are silent. However, the files are preserved by the retro community. If you are willing to learn how to use Nokia Care Suite, you can bring your Asha 302 to its ultimate form. nokia asha 302 software update 15.09 download