I need to enable ssl instead of http only in order to access freepbx on web.
It is beyond the scope of the help forum to instruct you in customization.
If you take a look at the packages installed, httpd server on CentOS you should be able to find plenty of articles on configuring SSL.
Can i have the whole procedure?
I told you there is no specific procedure and gave you guidance on where to look for information on the httpd web server included in the distro.
In fact after re-reading your message I have no idea how you installed FreePBX so my advice might not even be accurate.
I download ApacheConf
And it helps me configure ssl
yum install mod_ssl
Ward to the rescue,
Once you have that mod I would assume you can install your certificate.