Thanks for your advices. In fact I have tried to dissemble it. I prised off the red/blue discs, from the hole, I can see something like the screw hole, rather than an Allen key hole. I could not take it off using either a screw drive or Allen key. Perhaps I need a special key with screw on it to...
I have a dripping sink tap in my kitchen. It's a double-handle mixer tap. See the photo. I even could not dissemble the handle, perhaps a special key is needed to dissemble it? I saw a DORF brand on the body. Anyone knows how to fix it? Thanks.