I have run into the same problem with trying to uninstall CheckPoint Endpoint from machines that have problems upgrading, either it would get stuck in an endless loop trying to upgrade or it would upgrade, but some blades would be corrupted and get stuck trying to finish the upgrade. I then try to uninstall it, but it would either fail or not accept our password that we set, nor the default 'secret' password. TAC could not help us with this issue so we had to reimage the machines.
To answer your question, to change the uninstall password, open up SmartEndpoint and click on the Policy tab. Scroll all the way to the bottom and under Client Settings, click on 'Default installation and upgrade settings'. In that window, you can click on 'Client Uninstall Password' and there you would change the Uninstall Password.