First of all you need to install certbot:
yum install -y certbot
then stop your web server and take certificate.( Note , you should had valid DNS record for your domain):
certbot certonly --standalone
here you should write domain name of your server
To renew certificate add next in crontab:
echo "0 0,12 * * * root python -c 'import random; import time; time.sleep(random.random() * 3600)' && certbot renew -q" | sudo tee -a /etc/crontab > /dev/null
If you want to extend exist certificate ( for example you have domain.com and want to extend for www.domain.com) , stop your web server and do next:
certbot certonly --expand -d domain.com,www.domain.com --standalone
then start your web server again.
