Remove old kernels on RHEL v6.3

1. Keep at least two kernels for emegency cases to ensure your ass
2. Remove older kernels but consider step 1

you can remove kernels using rpm -e command

3. Install yum-utils package and apply
# package-cleanup --oldkernels --count=2  so that you can keep two  kernels.

4. vi /etc/yum.conf and modify installonly_limit parameter to limit the number of installed kernels