It is not possible to change the name of the mail account of your "UTokyo ECCS Cloud Email" (= Google Apps for Education's account). However, the name that is attached to the mail at the time of sending can be changed in the following way.
ECCS Cloud Email is utilized in cooperation with Google Apps for Education and the University's system (UTokyo Account/ ECCS Account). Therefore, there many be some things which you may not even if it is possible in a regular Google account, For example, you cannot change the "Name" in the interface of "personal information and privacy settings" on Google.