Go to the Zotero site and select the Download Now link..
....or just type https://www.zotero.org/download/ ... for Windows
Use the same link to download and install a browser connector - if needed, you can install connectors for more than one browser.
The Word Processor plugins are bundled with Zotero and should be installed automatically for each installed word processor when you first start Zotero.
Both the browser connectors and word processor addins can be added later from the Zotero Tools menu.
If you previously installed the Firefox versions of the word processor plugins into Zotero 5.0 or Zotero Standalone 4.0, you should uninstall them from Tools → Add-ons.
You can reinstall the plugins later from the Cite → Word Processor Plugins pane of the Zotero preferences. If you're having trouble, see Manually Installing the Zotero Word Processor Plugin or Word Processor Plugin Troubleshooting.