By the look of it just trying on mine the sliding portion push forward then curve none overlapping bit outwards - tease bottom and top out whilst sliding rearwards.
Careful not to chew up felt runner when refitting
My fixed one appears to be siliconed in place. May require a bit of harsh language and something to grip the window firmly with while pulling out, if sacrificing it why not drill a few holes and tie some string through it to act as a pulling handle?
