XF 1.5 Unable to Change http to https


My default installation of xenforo is with http, and now I want to change it to https. I've tried to edit "Board URL" from the option menu, also changed the license URL, but somehow <base href=" always sticked to http instead of https. Is there any cache need to rebuild to perform this action?

I use cloudflare so no need to install cert.

This should be nothing to do with the cert. If I view the html, the base tag still in http instead of https. It means $requestPaths.fullBasePath stays with http instead of https, doesn't respect "Board URL" configuration.