The email address of your contacts is BOMA's primary record and can not be amended.
You should add the contact's new email address as a new contact.
Unsubscribe the old record so that you can retain the history of email content they've engaged with. If that's not required, you can delete the old record.
Further Reading:
How to add contact/s that have unsubscribed from your BOMA database
Wolters Kluwer iFirm - How to export contacts to import into BOMA
What is a tag and how do I use them in BOMA?
How do I add a Tag to my contacts?
How do I upload a CSV file with new contacts into BOMA?