To be clear, none of the installers I've used have offered an 'upgrade' option as such.
What you need to do is wait until you get to the part when the installer looks for other systems on your PC. You then get a window (from memory) where the first option is to overwrite ubuntu with the new system. The second option is to install side by side, keeping the original OS in place. The third option is 'something else'. When you choose the third option, it takes you to the disk partitioning screen. On here, you must choose your original root mount point (if you only have the one drive then this is just the largest partition). You choose to mount it as '/' (root) and set the partition size (just leave it as it is) and the filesystem (most likely ext4 unless you chose to change it on your original install). Make sure to 'untick' the format box!!!!
The installer will then tell you that changing the partition size could take a while and ask if you want to continue. Choose yes. This bit always takes a couple of minutes for me in spite of the partition actually being the same size. The installer will then explain that all the system stuff (/etc /usr) etc etc will be overwritten and ask if you want to continue.
When you do, it will make a list of all your installed packages first, before it installs anything. At the end of the install it will try and re-install them. This bit often (always?) fails. However, as long as you use the exact username, PC name, and password that you used originally, all your stuff in your /home directory will be intact.
You might want to check out
https://help.ubuntu.com/community/UbuntuReinstallation