I want to upgrade Ubuntu and keep my existing files and apps. Is there a way to upgrade from Ubuntu 13.10 to Ubuntu 14.04 without having to download an ISO, put it on a USB and then install it and have it erase all my files and apps?
Ubuntu has already answered this question on their very own site.
Yes you can. You should check in the Software & Updates on the Updates tab for what versions you get update notifications. The other way is to download the ISO and make a bootable CD or USB dongle. When you boot the live CD and start the installation, you should be given the option to upgrade 13.10 to the 14.04 without erasing our files and apps.