It would be easier to move the PC downstairs temporarily for the setup using the cable (broadband is not so keen on long cables, especially if they are joined). Then disconnect the cable and use the USB upstairs (going through the setup for the USB though).
BTW, Ethel, not all modem/routers need to be set up with cable now. BT's new modem/router lets you set up wirelessly from scratch! However, I still prefer to set up using an Ethernet cable (less hassle/more reliable? [fo3nix might disagree?]).