When you update CentOS and there is a new kernel CentOS will install the new kernel but not remove the old kernel, this can become problematic when you have a very small /boot partition so it is a good idea to remove the old kernels that are no longer needed. Please note that if you have a large /boot partition it is not necessary to delete the old kernels. Before deleting any of your old kernels...
March 31, 2017
How To Remove Old CentOS Kernel images