It is a common problem on fortwos to have mysterious dampness in the boot, which may be attributable to water leaking through the rear windows. You should be able to see water marks at the base of the window frame if this is the case.

Use a very thin blade screwdriver or a knife to gently ease out the edge of the clips.

To remove the frame simply pull out the leading edge and the back edge in towards the window, and pull the whole frame down until it releases.

Grip the window on the outside (you can lift it gently at the back edge for a better grip) and pull the window towards the back of the car (it is only held on by one clip).

It is much easier to gently lever out this clip and slide it back into the window than it is to try to locate the window back on the clip!

Clean up the exterior frame, as there is no replacement seal available, and it is not really viable to clean the existing seal as it will fail again.
For a permanent and effective fix, evenly spread some silicone sealant around. Then press the window back into the frame firmly to locate the pin back in the tridion.

If you have spread the sealant correctly, there will be no residual smells in the car. Any odour from the exterior will soon go off.

Now have someone press firmly on the outside of the window whilst you relocate the frame inside; it will click into place.
For additional peace of mind you can squirt a small amount of sealant in the gap behind the clip.

Simply press the retaining clip back into place.