On TI-Nspire CX hardware revisions A-I, you can downgrade from the full OS 3.6 by flashing Boot2 3.1.0.16 through an USB/TTL interface and then installing Nlaunch.
On TI-Nspire ClickPad/TouchPad, you can downgrade from the full OS 3.6 by flashing Boot2 1.4.1571 through an USB/TTL interface and then either installing Nlaunch, either using the downgradeFix tool.
On TI-Nspire CX hardware revisions J and above, you can't downgrade from the full OS 3.6 for now.