If there is a 'cap' in the top, this can be prised out, gently, to expose a screw which holds the main tap top. This can then be tightened. Hope this helps.
davidj, August 2011
buy new heads its easy that way undo and replace you need socket set plumber opener
lal, August 2011
edit
I can answer this question...
How can I fix the Mixer taps bought a few months ago?