Have you tried getting the manual from their web page? http://www.sharp.co.uk/cps/rde/xchg/gb/hs.xsl/-/html/support--downloads.htm I obviously don’t have the model number of your microwave but you should find it here. If however you have one of those models that has a sealed unit light (one without a lamp cover) it would be best to contact a proper repair centre as it can be extremely dangerous to open up a microwave if you don't know what you are doing.
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Find the lamp cover - sometimes its outside the case, sometimes its inside. Remove cover. Unscrew lamp: fit new lamp.