If it is something like this:
http://www.spareszone.co.uk/CHIM60SS-myappliances-art10901-60cm-chimney-hood-stainless-steel/designair-20131858-lamp-holder.htm
it looks to be as simple as isolating the mains supply, inscrewing the old one, screwing on the new one and connecting the cable.
You find someone competent to mess with electrical things and ask them to do it safely as, going by your question, you don't appear to be.
XXX
December 2015
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