Tells how to replace a kernel with an older one. But it does not describe hot to boot an older kernel it the newer on does not work or crashes.
It is a bad mistake to think you'll have a running system in front of you. You might have a crashing system in front of you and you are forced to boot an old kernel. It should describe how to do this.
After you managed to boot you crashing system agein, you'll be able to replace the new, crashing kernel with an old one.