In the public wifi network, I can't download files with http links but I can use https links. How can I download a file served with http? A solution I have proposed is to convert the link from http to https but I have no idea how it's done.
It sounds like you are trying to get around a transparent proxy that is blocking things on normal unencrypted HTTP but not doing anything with HTTPS.
There is no simple way to "redirect" HTTP links to HTTPS purely from the client end. The server in question must support sending the content through HTTPS on their end, and from the client end you can't do anything or make the server do what it isn't configured to do.
Some sites redirect users that go to an
So you will need a proxy or similar outside of the network to accomplish what you are trying.
You need something like a reverse proxy to get insecure content and serve it over a secure connection.
Similar question (but about images): http://stackoverflow.com/questions/36381612/easy-reverse-proxy-for-serving-images-over-ssl
There's a number of answers there - but the short answer is that you need another server.
An alternative, which I ONLY recommend if you have an actual emergency, is to connect to Tor and download the file over Tor. The reason for not recommending this is that Tor is that it will needlessly strain the network and the file transfer will most likely be excruciatingly slow.
UPDATE: Another option is to use a VPN or a public proxy. However, again, this requires another server between you and the file.