First of all make sure you have downloaded Decompress.exe from http://www.gildor.org/downloads
Once installed. Drag the .UPK file or any other supported extension, onto the decompress.exe, if everything went fine then we should now see a new folder named unpacked in the same folder as the decompress.exe
Open it and look for your file, it will now be decompressed and safe to open with UE Explorer.
Alternative Method
If the above failed, then this either means that the package could not be opened by decompress.exe, probably because the game isn't supported or the package was compressed with LZO, if the latter is the case.
Make a shortcut to decompress.exe and open its properties. Add the following parameter to target "-lzo" with a space before.
Next add the package name including the extension next to -lzo with a space after.
For example: "decompress.exe" -lzo Core.upk.
Run the shortcut and check the unpacked folder again!
Once installed. Drag the .UPK file or any other supported extension, onto the decompress.exe, if everything went fine then we should now see a new folder named unpacked in the same folder as the decompress.exe
Open it and look for your file, it will now be decompressed and safe to open with UE Explorer.
Alternative Method
If the above failed, then this either means that the package could not be opened by decompress.exe, probably because the game isn't supported or the package was compressed with LZO, if the latter is the case.
Make a shortcut to decompress.exe and open its properties. Add the following parameter to target "-lzo" with a space before.
Next add the package name including the extension next to -lzo with a space after.
For example: "decompress.exe" -lzo Core.upk.
Run the shortcut and check the unpacked folder again!