If you only want to delete the installer, you can select it from the Trash, then right-click the icon to reveal the Delete Immediately… option for just that file. Alternatively, your Mac can delete the macOS installer on its own if it determines that your hard drive doesn’t have enough free space.

What happens if you delete Mac OS?

It is safe to delete, you will just be unable to install macOS Sierra until you re-download the installer from the Mac AppStore. Nothing at all except you would have to download it again if you ever need it. After install, the file would usually be deleted anyway, unless you move it to another location.

Can I delete old Mac OS?

No, they aren’t. If it’s a regular update, I wouldn’t worry about it. It’s been a while since I remember there was an OS X “archive and install” option, and in any case you would need to select it. Once it’s done it should free up the space of any old components.