If you want to just open the file and access it's contents you can use lots of editors, I use IZArc freeware. If you want something that will actually edit and make pak or pk3 files for modding download Pakscape.
Pakscape is a bit different though, you have to copy and paste the file into it to open it and if you want to edit a copied file you should drag it to desktop first, edit it, paste it in Paksape, then save in the appropriate file type that the game uses.
Of course, but you need not do that with Pakscape, just save it as a pak or pk3. One's better as an editor, the other as a modder. Many modders still use Pakscape even though it's an older tool.