Leaking Kitchen Tap. I have this tap in my kitchen (Atlanta Single Lever Tap PVD Stainless Steel

Question

I have this tap in my kitchen (Atlanta Single Lever Tap PVD Stainless Steel Atlanta Single Lever Tap PVD Stainless Steel) and it has two sections, and the top section leaks, and we have to leave it in a certain position otherwise the tap will slowly leak eveywhere.
Is it the washer? if so how does the tap come apart? Do you have to turn the water supply off prior to dismantling the tap?