How To Upgrade To Minimize Pain
# apt-get update
# apt-get upgrade
# apt-get autoremove
If you don't do the last two, things that are no longer needed will accumulate and waste disk space, including kernels, including in /boot. And if you are messing with kernel versions, you probably want to reboot to get on the latest one so that you don't accidentally auto-remove the kernel you are working with.