How do I manage the media in my iPod?

Carol has an iPod that's getting too full and wants to know how to get everything off it and allow you to put other music on it. Leo suggests using iTunes and it's sync feature. Set it up to only sync checked items. Anything that isn't checked will be removed. Some items, though, won't get unchecked in her iPod. Leo says that's not where you uncheck them. You uncheck them in iTunes. Here's how - https://support.apple.com/en-us/HT201593