If this removes OS 3.6 and leaves you with a 3.1/3.1 CAS system instead of a 3.6/3.1 CAS system, I recommend you leave it like that, because some 3.1 apps (like Vpatch and ThemeEditor) don't work on later OSes and if you do want to use them, you might run into bootloop issues when going back to 3.6.
If you still want 3.6 or 4.2 (there's no real reason for you to in my opinion), you can get the OS, use TNOC on it to remove Boot2, and possibly Examples as well (if you want to, it's just a waste of storage to keep Examples), rename the OS file to phoenix.tns, take your Nspire CX (while on CAS OS, if it isn't on CAS OS you'll replace the CAS with the non-CAS OS, which would make this whole tutorial useless), and delete phoenix.old.tns on the nlaunch folder. Put your phoenix.tns on that folder, reboot the TI-Nspire, and voila.
☆3.1/3.6 Switch Pack
☆TI-Nspire CX OS v3.6.0
☆TI-Nspire CX CAS OS v3.1.0
☆meXact/mCAS (exact calculations on Nspire)
☆KhiCAS (CAS on Nspire, somewhat buggy)