After finishing up all exams in lockdown, run Lockdown browser once more, without any active tests or assignments.
Quit the application immediately after starting.
Make sure you use the Respondus uninstaller in the Windows Control Panel (Control Panel -> Programs -> Uninstall a program).
After uninstalling, reboot your PC.
If you encounter any issues with things such as power options disabled, you can reinstall Lockdown browser, and repeat the process above.
The steps above should undo all system modifications Lockdown makes to Windows.