First of all: I am aware of the rule for "a" vs. "an" be it a noun or an abbreviation following (see When should I use "a" vs "an"?). So following this rule the answer would be "an SW".
But how about:
I need to perform __ SW update.
Will the rule still be valid if you consider that - in the context of SW=software - probably no one will ever read this as "an ess-double-ju update" but always read this as "a software update"?
What would you recommend here?