if you have the engine installed, is quite easy.
first, you need to check if the unpak executable is in your engine version. you have to be in the engine folder and then go to Engine\Binaries(your os)\
if in that folder there is a exe called UnrealPak.Exe you can continue.
open the cmd and type: “(your engine location)/Engine/Binaries/(your os)/UnrealPak.exe” (the file you wanna unpak) -Extract (the directory you want the result copy on)
example: “C:\Program Files\Epic Games\UE_5.3\Engine\Binaries\Win64\UnrealPak.exe” D:\PP3\Content\Paks\packchunk1003-windows.ucas -Extract D:\Extracted
it’s VERY important that the unrealpak.exe part be underlined
this method could be used for .pak, .utoc and .ucas files
note: if it tells you ‘Ran out of memory allocating x bytes with alignment x.’ you have to increase the virtual memory ( www.youtube.com/watch?v=TmEs3PBF8EA ) increase it in the disk you’re extracting the files on. the new value has to be the double of the file you’re extracting