According to Clement Lefebvre (the Mint project leader), the only danger is if you installed from a compromised ISO. Mint's download page was compromised and if you happened to have download a Mint ISO on Saturday, February 20th, that might have been a bad one.
To answer your specific questions:
1) First, if I already had Linux Mint installed on my computer and I simply upgraded from Rosa to Cinnamon via the Update Manager, am I ok?
Yes, you're fine. The only thing that was affected was the live CD images (ISOs) stored on their download server. Already installed systems wouldn't be affected. Upgrading through the Update Manager doesn't use an ISO image, it simply updates the necessary packages using the latest versions in Mint's repositories and those weren't affected by this.
2) If not, is there a way I can simply view when I upgraded my OS in the terminal?
Not relevant, see above. Upgrades are safe, it's only new installations made using a compromised ISO that are problematic.
3) If not, I have read the Linux Mint blog post about how to check if my ISO is compromised using the md5sum command, but in order to do that, I need to find the ISO file. Where would that be located on my computer?
We have no way of knowing. If you installed your system using an ISO, then you downloaded that ISO from a different system. Either on the same computer with a different operating system or a different machine. This isn't the sort of thing you could do unknowingly. ou would have had to i) download the ISO; ii) burn it onto a CD/DVD or copy it to a USB drive; iii) boot off of the DVD or USB and install your operating system.
So, the ISO, if it exists, would be wherever you have chosen to save it. Just like any other file you download from the internet. That said, the default name (which you might have changed, of course) is one of:
You can look for all of them using
locate. First, refresh your
locate database with (this can take a while):
locate --regex 'linuxmint.*iso'
If that returns nothing, the ISOs aren't on your system.