Go into a sprint store and tell them you want your phone updated, thats what I did. Unless you want to download the update yourself and do it but going to Sprint is just easier and takes about 45min.

 

Are you sure you want it?

I would consider rooting the 1.5 and putting a Custom 2.1 build. The stock 2.1 is pretty bloated with Sprint Crap and 3D performance is pretty much non-existent.

Once you go Stock 2.1, YOU CAN'T EVER GO BACK (not until someone at xda-developers finds a way to root it and modify the new boot loader).

-Mike