First, you will need to update the constituent's main profile page with the new email address. IIf the constituent has any email status showing above the email address, you will also need to remove the status by clicking on the eraser icon, then entering YES in the popup window and then OK. Click the Save button at the top of the page to save the changes.
Next, you will need to delete their old portal account. From the Portal Users grid, go to the constituent's record and click on the delete icon on the right side of the row. Click Delete to confirm the deletion in the pop up window.
Finally, a new portal account must be created. You can create a new portal account yourself for constituent using the new email address and a simple password. The constituent will receive a message asking them to confirm the new account, but you will need to send them their password. You can also send the constituent the portal login URL and have them sign up themselves with the new email address.