There's a teardown of the 505 that Igorsk posted
here. To replace the battery you will only need to go up to step 9.
Frankly, it all looks pretty straight-forward. Only one ZIF-connector needs to be detached and the battery itself has a proper connector. If you can replace the battery in an iPod you should have no difficulty changing this one. I'd recommend using a thin plastic spudger to work off the top and bottom panels so you don't scratch the finish.