You can change your user name (email address) from "User details" (top right corner, under your name).
Press the pen icon next to your email address.
Insert your new email address twice and press save.
You will get a confirmation message to your email. Please click "confirm" on the message you received.
Then lastly, just insert your password and click continue.
Your user name has now been changed. Please make sure your old email address is not being used somewhere else (for example for invoices).