You can edit an activist's email address by clicking on the "Edit Email" button from the edit page. Change the activist's email in the text box and click "Save Changes" to update an activist's email address.
If the new email address is one we have not encountered before (and thus we have no pre-existing information on), your change will be made. If your group is part of a Network, the change will be made throughout the entire network.
However, if the email address is one we have some information on we will ask you to merge the two activists instead, letting you choose which information you want to keep. See here for more on merging.