What's the best way to acquire source art?

I’ve bought several art packs on the UE4 marketplace, but I would like to re-texture some of these assets, as well as make small changes to geo. What’s the best way to extract these files from the .uasset?

While inside the editor, right click on the mesh/ texture, look for Asset Actions, choose Export and then choose the location to export those file to. Generally, you only have to look for Export option.
Also, check the legal terms carefully, i usually have my head explode whenever i have to deal with those, and i have to, unless i want my wallet to explode.