If you're in Commissioner mode then there are two steps:
1) Use the player editor to put the players back on their original teams. Once in the player editor, scroll to the very bottom on the left. That's where you'll see the league and team dropdowns.
2) Go into the player History panel, then then near the top right, click on Edit History. At that point you can just remove the entry that indicates the player was traded.
At that point it's like the trade never happened. And then repeat for the other players involved.
Good luck. Come back here and ask questions if it's not clear, or if I've totally misunderstood what you're trying to accomplish... which is always a possibility
