Backup old kernel

Every time, when a new kernel is installed, the last installed kernel will be backed up in a directory who is called kernel.old. So it is important to remember to save a copy of usable kernel in case the newly-built kernel can't work. For example:

# mv /boot/kernel.old /boot/kernel.good    
# mv /boot/kernel /boot/kernel.bad
# mv /boot/kernel.good /boot/kernel

During boot, you can press 5 to select which kernel to run:

References:
FreeBSD: reverting to previous kernel;
If Something Goes Wrong.