Go to your team page and click on the Details link that appears next to each trade offer. You will then be able to accept it, reject it, or send a counter-proposal to the manager who proposed the trade. If there are no details, the manager proposing the trade has canceled the offer.

Once a trade proposal has been accepted, the teams involved in the deal can no longer cancel the trade. The trade is then put to a league vote in Public Leagues, or to the commissioner or a league vote in Custom Leagues.