Best answer by Bumblebee
If your MyOVO account has gone livehttps://my.ovoenergy.com/login you can edit the email under "profile".
If its not up and running yet you can call customer service (Note, you'll be amazed how quick they answer having switched from Scots) on 0330 303 5063, they may need your partner there to give consent to the change.
Or better still, contact OVO on their Facebook page - just have your account number, date of birth and postcode to hand :)
Hope this helps, if there anything else I can help with along the way please don't hesitate to get in touch,