1- Open the Mac Terminal accessing to the Spotlight icon.
2- Start typing terminal in the search box.
3- Once it find the app, double click in the Terminal icon to open the terminal window.
4- Type sudo su - root to become a root user.
5- Type defaults write /Library/Preferences/com.apple.networkd tcp_connect_enable_tls13 1 command and press Enter.
6- Restart Safari.
Note: If you are using Safari version 7 or greater, TLS 1.1 and TLS 1.2 are automatically enabled.