That's not the problem, guys. She's trying to update to 6.1. She wouldn't have been able to start the update if she was trying to install 5.x. She didn't mention SHSHs, so it's likely she's just tying a regular update to the latest firmware; she just didn't know the version number.
Sarah, again, please post your exact error code. It's difficult for us to provide help without that. Until I know that, I'll offer the standard troubleshooting steps. Try a different USB cable, different port on the computer, reboot the computer and the phone, and temporarily disable your antivirus program. If that doesn't help, try updating on a different computer.