Note: to upgrade to the current version of Ubuntu, you don't have to download the ISO again, you can do an in-place upgrade: press Alt+F2 and type in
update-manager -d
This launches the Update Manager with a button for the new version. It'll download hundreds of megabytes and do the upgrade, which may take a good bit of time, depending on your system. This way preserves all your files and settings. But, as noted, that's not a guaranteed fix for your problem.