Before removing the watertank you first have to empty it. To make sure it is,  slide the watertank as far as possible out of the aircooler. First you have to turn the knob anti clockwise.

After that you can pull the watertank out of the AT-5450 aircooler as far as possible.

In the watertank you can see a small white plug that you can remove by pulling it (see red arrow)

Make sure the aircooler is placed somewhere the drained water cannot cause damage. (outside or maybe in the bathroom) It is also possible to place a low tray underneath.

When the plug is pulled out, the water can drain out of the tank. 

After the watertank is completely empty, you can tilt de aircooler. At the bottom you see a square plastic plate with a screw in the centre. Unloosen the screw and remove the plastic plate.  The watertank can now be removed.