Hi Michelle,
My personal preference for a one-off like this is to edit it in the CRM like SFDC - it's just a bit faster and easier. That being said, it's certainly feasible in Marketo.
You can edit a record's email address a few ways within Marketo. I'd recommend pulling up the full lead record in the Marketo database and editing it there.
First, find the record you want by doing a quick search
Double click on your record, and go to the Lead Info tab of the next screen. You can change the email address here.
The record will auto-save like an email or landing page. Close the tab and you're done!