Go to http://support.dlink.com/products/revision.asp and check your hardware revision, then click the appropriate link for the upgraded firmware.
If you have hardware version A1, looks like you are out of luck, they only have 1.10 up for it.
Under hardware version B2, they are up to 2.24. BestBuy is currently selling the B2 version, although you cannot tell until you open the box.

(I'm thinking that putting 2.24 on A1 would be a recipe for turning your router into a brick)
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Added
Oops, looks like the link above requires some session data indicating the product ID or it bombs.
Try this instead:
http://support.dlink.com/downloads/
and pick DIR and 615 for the product, then you'll get to the right place. Sorry about that.