Ready-to- use external hard drives don't have screws, the upper and lower parts of the outer shell are held together by concealed clips around the edge. If you turn the case upside down you should see a seam all the way round. You have to carefully prise the seam open all the way round until you feel or hear the clips release themselves. You should then be able to separate the two halves of the shell.
In some cases it's necessary to break the shell to separate it, or you'll be lucky if you manage not to snap at least some of the clips. Basically they aren't made to be opened up easily, the manufacture regards the whole thing as a sealed unit.
There's a lot to be said for making your own external hard drive by putting an internal hard drive inside a third-party, re-usable enclosure which can be opened and closed very easily. They are sold on Amazon and many other online PC accessory stores.