to update Nero 5, you just need to download the updater program from Ahead's website (or from anywhere else) and double-click the file.
this is an update and is free and needs no serial key or anything.
an upgrade is to change from Nero 5.5 to 6.0 and for that you need to purchase a key or the retail package.
if you purchase a Nero 6 serial key it will let you use the Nero 6 program but will not update your Nero 5.5
One Ring to rule them all, One Ring to find them,
One Ring to bring them all and in the darkness bind them
In the land of Mordor, where the Shadows lie
M.C. Escher - Reptilien