Correction Herman:
To do so, on the screen that shows the certificate warning (and no option to bypass) you type: 'badidea' (including excluding the '-quote signs) and you will bypass the warning page.
Don't type it into the address field by the way. Just 'invisibly' type it while the page is open.
@Herman Robers wrote:
The instructions that Colin sent include how to upload the new certificate via the CLI, so you don't require to use the web interface.
If you prefer to use the web interface, and you are a Google Chrome users, there is a trick to bypass the security warning in this case. To do so, on the screen that shows the certificate warning (and no option to bypass) you type: 'badidea' (including the '-quote signs) and you will bypass the warning page. As far as I know, this only works with Chrome; but it prevents you from reverting back the WebUI to HTTP (which I even don't know if it is possible; it is a bad idea though).