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The life cycle of money
Echoing JohnF, and assuming you mean the physical, rather than abstract meaning of money?  The abstract concept obviously isn't replaced (unless the currency is discredited, or like the creation of the Euro which saw local currencies abandoned). The actual bits of paper are regularly collected, shredded (into itty-bitty-bits) and destroyed.  Coinage tends to last a lot longer, but it also collected and melted down eventually. Depends on the country, though.  No doubt, many people who took a gap year to go travelling in points diverse came across countries where the money is a sort of brown-grey smudge you hold with care in thick wadges. The more modern economies replace paper money on a dedicated cycle (around three years according to Wikipedia, anyway).