Since I’ve been asked, I’ve done a short video showing how to open an Android mini PC, namely Droid Stick A2. As most casings are based on similar design the same method can be used for many other mini PCs that comes with a plastic casing. Some other mini PCs with a metallic casing, such as Hi802, need to be opened with a different, and IMHO easier, method as you just have to remove 2 screws.
For Droid Stick A2, all you need is a flat head precision screwdriver. First look for small spaces between the two parts of the casing, insert the screwdriver where the largest space is, and gently lift it up. Watch the video below for a demo.