The contract of the temporary marriage is ended when its set time is ended or by the man waiving and forgoing the remainder of the time. This can be by stating: "Wahabtu ... (so-and-so) ma tabaqqa min muddat iz-Zawaj," mentioning the wife's name in place of "so-and-so". This means, "I waive for so-and-so the remaining time of the marriage." It is permissible to state this in any other language. After you did so, you can inform her by chatting online or by any other means.