Roger, if you're on the web version of Duolingo, you can go to the Settings via your icon in the top right corner of the screen and change your email address there.
On the mobile app (at least the Android version), you can get there via your profile (by tapping the face icon at the bottom of the screen) and there you tap the cogwheel in the top right corner.
In UK English "sweet" is commonly used instead of dessert so why doesn't Duo accept "sweet" for postres. Furthermore if you ask any waiter in Spain for a sweet or the sweet menu he always knows you mean postres. Another substitute word in UK English is "Afters" but again this is not accepted.