The first thing you need to think about updating a kernel is that “why do you need a kernel update”.
Many people think about updating kernel for the vulnerabilities that may not get fixed by updating the kernel.
Because –U option will delete the previously installed kernel version which we may need in case of any unsuccessful update.
Replace the version with the exact ones you are installing.
I have firefox 3.0.12 on my redhat 5.8 and I'm trying to update it.