Woah this is awesome!

First: Hold down [doc]+EE+enter
Second: Press the reset button
Third: Wipe the OS
Fourth: Connect to computer and send the 3.1 os over
Fifth: Install ndless

Sorry to bump an old thread, but I didn't want to start a new topic when it's already been done.
I have a TI-Nspire CX CAS with OS 3.2, looking to downgrade it to 3.1

This may seem like a silly question, but how exactly do you wipe the OS completely?
Followed the first two steps and nothing happened.  I already have the 3.1 file on my computer, I just need to know how to put it on my calculator to install it.

Yes, basically what Lionel said.  The installer should see all your partitions.  Just select the partition you want to install Mint on (make sure it's not the one Windows is on, i.e the 200GB one).

Also, note, Linux uses a different notation of drives and partitions.
Your main drive would most likely be something like /dev/sda.
Then the main partition would probably be like /dev/sda1, the next /dev/sda2, and so on.  Each partition on the drive /dev/sda.
So if you were to plug in an external drive, it would probably show as /dev/sdb.

It might also be hdx instead of sdx, but the concept is the same. 
After installation, you can remove the install media (USB, CD, or whatever) and reboot.
You should get the grub boot loader screen which, as the name suggests, will you allow you to choose which OS to boot into. 
Have fun with Linux!

How are you all?
I'm new here (hence this post in the first place).

I suppose I should introduce myself.  I love math, computers, and science.

Today I received my TI-Nspire CX CAS calculator, as my previous TI 83 broke.
I look forward to learning how to program it.

Currently I am learning C++, though I'm still pretty much a beginner with it.

What do you guys like to do?