In English we have only an open e [ɛ] heaven, the closed e [e] doesn't exist. The difference however is easy to hear if the two sounds are directly opposed.
The production of a vowel is always difficult to describe because it depends only on the resonance body formed by the mouth, in other words of the position of the tongue, the lips and of the degree the mouth is open or closed. But you can try to pronounce heaven without moving your chin. The result will not be very English, but very Portuguese.
(If you speak Spanish: In Spanish we have only a closed e as in energía, egresado, error etc.. If you speak French: In French we have both of them. If there is accent d'aigue, énergie, it is pronounced close. (Attention! In Portuguese the accent d'aigu indicates that the e is open! If you have an accent grave, it is pronounced open, grève.)
There is a innerbrasilian and innerportuguese debate whether the e should be pronounced close or open. The word obeso (obese) can be pronounced for instance with an open and a closed e. There is no need to have an opinion concerning this issue, but it is useful to know what the discussion is about. (This author would say that the discussion is a little bit theoretical, because between the open e and the closed e you have a lot of different phonems or sounds. Who can really say that the open e in cat, flat, tax are really pronounced the same way.)