There are a couple of different cases and without knowing for sure, I will try to assist. NOTE: You Do this at YOUR OWN RISK. Be gentle and patient. And remember that plastic can bend and stretch only so far.
Needed tools You do NOT want to use screwdrivers but flat wide thin spatulas about 1-2 cm wide at the tip. You want two to three.
Think in terms of the four sides around the screen. Start with the long side where there is not a switch or port opening
Technique: Gently slip the first and thinnest spatula into the crack between the screen front and Plastic Back, Insert this first spatula in the middle of the case unit. Pull back gently enough to insert the second spatula besides the first. repeat with the third spatula.
On most cheaper cases,there are ledge type fingers and shelves holding the case together. You may not need to remove the entire top. You only need to separate the fingers from the shelfs and create a gap which the MicroSD card can slide out. Remove the first spatula to create that gap to slide out.
This would be true if you can hear the card sliding around. Otherwise, you have to remove the entire top half
Remember these four things
1. The top is thin plastic and can crack easily if too much pressure is applied
2. The LCD screen is fragile and if broken, you have a expensive piece of plastic electronics
3. The buttons many times just rest in their holes and not attached to the top. You might want to tape them up to the case so they remain in place.
4. Be especially careful around and near buttons or input sections. They do not like to be pushed with the spatula.