"Load Program Guide...", not "Scan for channels...".
If I understand you correctly, you have a channel, say, channel 5, that actually shows NBC, but the guide says it's something else. There must be another channel in the guide that says it's NBC right? When you run Load Program Guide, using one of the XMLTV options, you will reach a step where you match your channels with XMLTV IDs. There you can match your "channel 5" with the XMLTV ID for the NBC station.