How do I unpack a cooked/compressed package?

I have some cooked & compressed packages (upk) which I want do examine. Well I saw one doing this (with the same files) successful. TL:DR; He won’t tell me how.

Well I’m not sure which information I can provide since I’m not very familiar with UE3/4.

Anyway I’ve tried the popular umodel from gildor which won’t extract anything with the reason unkown compression flags.

When trying to open them with the UDK Content Browser the package stays empty, even when I fully load them. Any attempts to export .. them end with the requested operation cannot be performed on a cooked package .

I would appreciate any ideas / tips,tricks.