This is very easy task,you need yum-utils package.

1. Check Installed Kernels

rpm -q kernel
kernel-2.6.32-279.el6.x86_64
kernel-2.6.32-279.2.1.el6.x86_64
kernel-2.6.32-279.5.2.el6.x86_64
kernel-2.6.32-279.9.1.el6.x86_64

2. Delete / Remove Old Kernels

##CentOS,Red Hat (RHEL)
yum install yum-utils

##Package-cleanup set count as how many old kernels you want left
package-cleanup --oldkernels --count=2

3. Make Amount of Installed Kernels Permanent

Edit /etc/yum.conf or /etc/dnf/dnf.conf and set installonly_limit:installonly_limit=2