If you're using Fedora, then just find an RPM for the older kernel (check the mirror sites) and install it as normal (rpm -ivh). Then when you reboot, select the new kernel at the GRUB display. You can change the default setting in /etc/grub.conf to always boot the older kernel.
One question -- why do you need an older kernel. Newer kernels have things like security updates and bug fixes that you shouldn't skip out on if at all possible.