Hi,
In order to reconfigure the platform for HTTPs, you need to re-run kaltura-base-config.sh and kaltura-front-config.sh, for nginx, the script is kaltura-nginx-config.sh.
kaltura-base-config.sh will prompt about the service URL and should input a URL with "https:///", kaltura-front-config.sh and kaltura-nginx-config.sh will each prompt for the location of the cert, key and in the case of Apache [kaltura-front-config.sh], CA files.
The certs should be standard x509 ones, which both Apache and Nginx expect, nothing special about Kaltura in that regard. You can get valid certs for free from http://letsencrypt.org.