To move to the latest development kernel say:
raspberrypi$ sudo rpi-update
rpi-update will load a kernel from github.com/Hexxeh/rpi-firmware. If you know the commit you can pass that as a parameter to load a specific kernel.
To move back to a production kernel say:
raspberrypi$ sudo apt-get install --reinstall libraspberrypi-bin libraspberrypi-dev libraspberrypi-doc libraspberrypi0 raspberrypi-bootloader