refer to:https://letsencrypt.org/

1. SSH into the server

SSH into the server running your HTTP website as a user with sudo privileges.

2. Install snapd

You’ll need to install snapd and make sure you follow any instructions to enable classic snap support.

Follow these instructions on snapcraft’s site to install snapd.
Installing snap on CentOS
Snap is available for CentOS 8 and CentOS 7, from the 7.6 release onwards. It’s also available for Red Hat Enterprise Linux (RHEL) 7.6+ (see Installing snap on Red Hat Enterprise Linux).
The packages for CentOS 8 and CentOS 7 are in each distribution’s respective Extra Packages for Enterprise Linux (EPEL) repository.
If you need to know which version of CentOS you’re running, type:

cat /etc/centos-release

If you don’t already have the CentOS repository added to your distribution, it can be added as follows:
The EPEL repository can be added to a CentOS 8 system with the following command:

sudo dnf install epel-release
sudo dnf upgrade

Installing snapd
With the EPEL repository added to your CentOS installation, simply install the snapd package:

$ sudo yum install snapd

Once installed, the systemd unit that manages the main snap communication socket needs to be enabled:

$ sudo systemctl enable --now snapd.socket

To enable classic snap support, enter the following to create a symbolic link between

/var/lib/snapd/snap

and

/snap

$ sudo ln -s /var/lib/snapd/snap /snap

Either log out and back in again or restart your system to ensure snap’s paths are updated correctly.
Snap is now installed and ready to go!

3. Ensure that your version of snapd is up to date

Execute the following instructions on the command line on the machine to ensure that you have the latest version of snapd .

sudo snap install core; sudo snap refresh core

4. Remove any Certbot OS packages

If you have any Certbot packages installed using an OS package manager like apt, dnf, or yum, you should remove them before installing the Certbot snap to ensure that when you run the command certbot the snap is used rather than the installation from your OS package manager. The exact command to do this depends on your OS, but common examples are

sudo apt-get remove certbot
sudo dnf remove certbot
or
sudo yum remove certbot

5. Install Certbot

Run this command on the command line on the machine to install Certbot.

sudo snap install --classic certbot

6. Prepare the Certbot command

Execute the following instruction on the command line on the machine to ensure that the certbot command can be run.

sudo ln -s /snap/bin/certbot /usr/bin/certbot

7. Choose how you’d like to run Certbot

Either get and install your certificates…
Run this command to get a certificate and have Certbot edit your Nginx configuration automatically to serve it, turning on HTTPS access in a single step.

sudo certbot --nginx

Or, just get a certificate
If you’re feeling more conservative and would like to make the changes to your Nginx configuration by hand, run this command.

sudo certbot certonly --nginx

8. Test automatic renewal

The Certbot packages on your system come with a cron job or systemd timer that will renew your certificates automatically before they expire. You will not need to run Certbot again, unless you change your configuration. You can test automatic renewal for your certificates by running this command:

sudo certbot renew --dry-run

The command to renew certbot is installed in one of the following locations:

/etc/crontab/
/etc/cron.*/*
systemctl list-timers

9. Confirm that Certbot worked

To confirm that your site is set up properly, visit https://yourwebsite.com/ in your browser and look for the lock icon in the URL bar. If you want to check that you have the top-of-the-line installation, you can head to  https://www.ssllabs.com/ssltest/.
check you site’s https://at SSL Labs