Locate the water tank connector assembly on the back of the rest of the coffee machine.

Remove its gasket by lifting the gasket upwards using a spudger or a plastic opening tool or your hands.

The gasket is not glued and can be removed easily.

Then insert a new gasket in the water tank connector assembly using your hands.

This gasket is a lip seal, so make sure to put the new gasket in the right position as shown in the 3rd picture.