I need to uninstall and reinstall Ubuntu, but when I open it, the only options that come up are Demo mode and Learn more, nothing about Uninstalling.
You can uninstall Ubuntu doing a new install. During this new install, you overwrite the old install formatting the old partitions and you replace their old content with the new installation.
Of course, consider doing a backup before.
Well Firstly there is no Option like UNINSTALL UBUNTU anywhere in Ubuntu universe , its Remove.
You can easily Remove your Current Ubuntu Installation by Using LiveCd or LiveUSB to Install a Fresh New Ubuntu OS.
Just follow the same steps you followed while installing your old ubuntu , just Search and Select the FORMAT option To the Old Ubuntu Partition drive. And proceed to new installation on the same Partition Drive or other drives as you wish.
NOTE : Do backup your important data from that partition , before proceeding. You can also copy your files using the Live Media of Installation. Also if you want same applications , Packages from the Old Installation , just follow this or this before New installation.