There are no detailed instructions as it is viewed as a technical job. But, it's not too hard.
You will need a BR2330 or CR2330 battery. On an H8000 you will see where it goes as soon as you remove the top of the case.
An H8000FW is harder cos it's under the FireWire board.
Look in the manual for "Storing and Loading Setups" before you start. Presets must be saved individually. Sigh.
If you have an H8000 and are technically competent you could consider changing the battery with the power on AT YOUR OWN RISK. This will preserve the setup and presets. Don't try it on an H8000FW – too difficult.