Finally i managed to open the ZEN 240IC.
I will be brief now because i am very tired and i managed to snap 1 connector from the ribbon cable.
So to open the PC you need to remove FIRST the small black plastic cover from the bottom of the screen. This will reveal some screws.
In order to do that i used the hair dryer to heat it a bit because the adhesive is quite strong on it.
After i removed that i used 2 suction cups ordered from Ebay to gently pull the top corners of the screen. I used 2 cups, 1 on each corner.
This requires patience as i struggle a bit to be able to pull the top part of the screen.
After the screen is detached you have some cables you need to remove in order to completly remove the screen.
One of the cables was the ribbon one from the motherboard to the screen...this was my luck.
Inside the HDD is clearly visible on the right side and without removing the motherboard there is no trace of another sata slot or PCIE M2 ssd or anything else.
I have just swapped the HDD with a SSD and that was it.
Hope this info is helpful to someone and please take good care when you remove the cables because now for me is a mission trying to get a new one from ASUS or to repair this broke one.