How do I clean the WV 6 Plus N Window Vac dirty water tank_ _ Kärcher UK

The WV 6 Plus N is Kärcher’s most advanced window vac that effortlessly sucks up moisture leaving flat surfaces sparkling clean and streak-free. The new silicone lip technology ensures increased durability and flexibility to ensure cleaning from the upper edge to the ground without interruption. Boasting an impressive 100 minute battery life, the WV 6 Plus N can clean up to 300m² in a single charge including windows, mirrors, tiles, shower screens, spills and even condensation.