There are two ways to 'undo' your trading activity, but they only work prior to the game lock-outs for the players involved in the current live round, meaning 'undoing' gets more complicated as the games of a round progress.
Reverse Trade button allows you to neutralise a specific trade. Select the trade (or both) you wish to reverse and click the button. Note, this may not be accepted if reversing would make your team invalid (e.g. you made two trades and reverse one that takes your over your slary cap).
Roll Back Your Team button reinstates your team to exactly what it looked like at the end of the previous lockout period. Clicking this button reverses any trades you made since the lockout was lifted and resets any adjustments made to Captain, Vice-Captain, Emergency selections and player positioning. Roll back only works up until the first lock-out of each round as once the first lock-out starts your C and VC choice is set.