Go to Downloads, click "Edit", select your zip file, submit. The name of the mod remains the same, but the version number must be different, and of course, in info.json, "factorio_version" must be set to "0.18" instead of "0.17" for the latest version. Only the version you uploaded last will be shown on the mod portal (users can go to your mod's Download page and download older versions directly), but it's different with the in-game mod manager: People who are using Factorio 0.17.x will only get to see versions of your mod where factorio_version is 0.17, people using Factorio 0.18.x will only see the versions for 0.18.
Regarding the "unknown key" bug, apparently you haven't localized your mod yet, or you missed some keys. In case you haven't done so already, you should read the Localisation tutorial. Also, you could make things much easier for your users if you add a changelog to your mod and always keep it up to date.
By the way: writing a changelog file that can be parsed by the in-game viewer is difficult, but not impossible. Syntax requirements are very strict, so you always should try before uploading that your mod's "Changelog" button in the game's mod manager is active -- otherwise check factorio-current.log for an error message, fix the bug, and try again until it works!
