HTTPS Redirection is an Extension for phpBB 3.1
Clone into phpBB/ext/ernadoo/ext-https-redirection:
git clone https://github.com/ErnadoO/ext-https-redirection.git phpBB/ext/ernadoo/httpsredirection
Go to "ACP" > "Customise" > "Extensions" and enable the "HTTPS Redirection" extension.