I'm not sure what you mean by "hard boot", as far as I know there's no such thing since Rockbox automatically shuts down rather than "sleeping" the player. If you're trying to boot the device in disk mode when connected to a computer you hold select while turning it on (with the USB cable connected to it and the computer, of course.) If you're trying to boot the Original Firmware (provided you installed Rockbox correctly and still have the original firmware binary,) you hold the back button (the arrow) while turning on the player until you see text printing on the screen, then it will load the original firmware binary rather than the Rockbox binary.
Replacing the battery is actually rather simple, all you need to do is find a replacement (I recommend either checking Ebay or searching on Google for "iriver H10 20GB replacement battery") and make sure you have a size 6 security torx driver (some places will ship one with the battery, but just in case make sure you have one available.) Once you get the battery, you use the torx driver to open the H10's case, then you'll need to use a flat edge of some sort to pry the back aluminum cover off the back piece so you can remove the old battery (after making sure you disconnect it from the board, of course) and put the new battery there. Then it's a simple matter of snapping the aluminum cover back in place, making sure you connect the new battery to the board, and putting the player back together.