There are 2 screws which have to be removed, but the cover still sits firmly in place because of small plastic catches around the rim of the cover.
These catches are engaged in small recesses around the rim of the housing. A small screwdriver has to be used to carefully pry those catches out of its recesses and the cover can be removed. So, I've upgraded the RAM with additional and compatible 2GB DDR3-1333 memory module (Skill). Works fine.
The same procedure of removing the memory cover works on series 5750 and 5742 of Acer Aspire, because the chassis seems to be the same.