First of all, when Yum installs a new kernel in Fedora Core, it doesn't remove the old one; so you can always choose to boot an older kernel in the menu when you boot up.
What kind of problems are you having with the new kernel? If you use any separate kernel modules like NTFS or 3D graphics card drivers, you will need to install them again for the new kernel since they are compiled for specific kernels.
|