http://www.stevejenkins.com/blog/2010/08/renewing-a-self-signed-ssl-certificate-on-fedoracentos/
Verify the filenames, locations, and the directives of your web server's SSL configuration by doing: grep SSLCertificate /etc/httpd/conf.d/ssl.conf
You should get something like:
# Point SSLCertificateFile at a PEM encoded certificate. If
SSLCertificateFile /etc/pki/tls/certs/localhost.crt
SSLCertificateKeyFile /etc/pki/tls/private/localhost.key
# Point SSLCertificateChainFile at a file containing the
# the referenced file can be the same as SSLCertificateFile
#SSLCertificateChainFile /etc/pki/tls/certs/server-chain.crt
you can change how long you want the certificate by by changing -days 1460 365= 1year 1460= 4 years
Type the following as root: openssl req -new -days 365 -x509 -nodes -newkey rsa:2048 -out /etc/pki/tls/certs/localhost.crt -keyout /etc/pki/tls/private/localhost.key
Answer the questions as they are presented to create your new certificate files.
Type service httpd restart to restart your web server and tell it to use the new certificate files.
You're done!