To display the current version of Linux (kernel) running on your system, run this command.

# uname -sr

Linux 3.10.0-327.10.1.el7.x86_64

List All Installed Kernels on System

You can list all kernel images installed on your system like this.

# rpm -q kernel

kernel-3.10.0-229.el7.x86_64
kernel-3.10.0-229.14.1.el7.x86_64
kernel-3.10.0-327.3.1.el7.x86_64
kernel-3.10.0-327.10.1.el7.x86_64

Removing Old/Unused Kernels on CentOS/RHEL

You need to install yum-utils, which is an assortment of utilities that integrate with yum to make it more powerful and easier to use, by extending its original features in several different ways.

# yum install yum-utils

One of these utilities is package-cleanup which you can use to delete old kernel as shown below, the count flag is used to specify the number of kernels you want to leave on the system.

# package-cleanup --oldkernels --count=2

Removing Old Kernels
Loaded plugins: fastestmirror, langpacks, product-id, versionlock
--> Running transaction check
---> Package kernel.x86_64 0:3.10.0-229.el7 will be erased
---> Package kernel.x86_64 0:3.10.0-229.14.1.el7 will be erased
---> Package kernel-devel.x86_64 0:3.10.0-229.1.2.el7 will be erased
---> Package kernel-devel.x86_64 0:3.10.0-229.14.1.el7 will be erased
--> Finished Dependency Resolution

Dependencies Resolved

===============================================================================================================================================================================================
Package Arch Version Repository Size
===============================================================================================================================================================================================
Removing:
kernel x86_64 3.10.0-229.el7 @anaconda 131 M
kernel x86_64 3.10.0-229.14.1.el7 @updates 131 M
kernel-devel x86_64 3.10.0-229.1.2.el7 @updates 32 M
kernel-devel x86_64 3.10.0-229.14.1.el7 @updates 32 M

Transaction Summary
===============================================================================================================================================================================================
Remove 4 Packages

Installed size: 326 M
Is this ok [y/N]: y
Downloading packages:
Running transaction check
Running transaction test
Transaction test succeeded
Running transaction
Erasing : kernel-devel.x86_64 1/4 
Erasing : kernel.x86_64 2/4 
Erasing : kernel-devel.x86_64 3/4 
Erasing : kernel.x86_64 4/4 
Loading mirror speeds from cached hostfile
* base: centos.mirror.snu.edu.in
* epel: repo.ugm.ac.id
* extras: centos.mirror.snu.edu.in
* rpmforge: kartolo.sby.datautama.net.id
* updates: centos.mirror.snu.edu.in
Verifying : kernel-3.10.0-229.el7.x86_64 1/4 
Verifying : kernel-devel-3.10.0-229.14.1.el7.x86_64 2/4 
Verifying : kernel-3.10.0-229.14.1.el7.x86_64 3/4 
Verifying : kernel-devel-3.10.0-229.1.2.el7.x86_64 4/4

Removed:
kernel.x86_64 0:3.10.0-229.el7 kernel.x86_64 0:3.10.0-229.14.1.el7 kernel-devel.x86_64 0:3.10.0-229.1.2.el7 kernel-devel.x86_64 0:3.10.0-229.14.1.el7

Complete!

Important: After running the above command, it will remove all old/unused kernels and keep the current running and old latest kernel as backup.

Src: https://www.tecmint.com/delete-old-kernels-in-centos-rhel-and-fedora/