You can setup https protocol here: Admin block > tab "Common" > section "Server configuration" > item "Server settings" > group "Server URL settings" > field "Server protocol" and "Server port". For https protocol in field "Server protocol" type "https://" value, and in field "Server port" type "443" value. Also in field "Forced server URL settings" you can choose "Yes" value, if yet https protocol don't work.
Also You can setup https protocol via database. Go to forum database, and choose table "config" (or "[prefix]_config", if you use prefix, e.g. "phpbb_config", if you use default prefix). Are in "config_name" field/column option "server_port" change from "80" to "443" value are in "config_value" field/column, and also option "server_protocol" change from "http://" to "https://" value.
