You have to first, install a really good virus and malware checker, then visit that very questionable corner of the web, locate, download and install a sleazy editor!
Actually, I don't believe you can or would really want to directly edit these files. I believe they are used within RoboHelp to provide the skin for a HTML5 Responsive layout. So you edit them by manipulating things inside of RoboHelp. Then you can export them and probably import them into other projects.
LOL. RoboHelp is a sleazy editor?
SLZ's are just zip files with a different extension. You can extract it like any archive. I regularly just thrown in updated images or .slp files usins 7-zip.
If you want to edit beyond the capabilities of the built in editor, don't open the .slp files in RoboHelp! That will mess up the layout. Use Dreamweaver/Notepad++/something else instead.
Hopefully Willam, you weren't interpreting my answer to suggest that one simply use the RoboHelp Editor to open a SLZ file as if it were any old topic.
I think it's any screen layout that has been exported from RH. If you rename it to .zip you can open or extract it like a normal zip file. I haven't tried this so I don't know what the consequences might be. I think you'd be better off importing into RH and editing the files that RH extracts, then re-exporting if you need to.