If you would like all ".html" files to be parsed through SSI without having to rename them to ".shtml" you can add the below line to a ".htaccess" file (found usually in the top level of your "public_html" folder):
AddHandler server-parsed .html
If you do not have a file called ".htaccess" (minus the quotes) you can simply add one using your cPanels' File Manager. To check to see if you already have a ".htaccess" file make sure you have selected "show invisible files" when you go to your file manager as files that start with dots "." are hidden.