First off, if you want all of them sunken, you usually have to take out the rubber seals, at the cost of water resistance. This would also be a reason to check out that one screw since it might be missing its seal.
If you want to do it yourself, you need to take off the caseback screws and then unscrew the little screw nuts with a special tool...it's a bit of a bitch. Here's the whole screw construction: