Simply double-click the downloaded file to install it. So, download this specific 32-bit version of iTunes from here. Keep in mind, the 64-bit version does not work properly on Chromebook and displays a black window. If the particular app you want to download does not offer an older version, see this utility to find an alternative app: Find iOS Apps Compatible with Older Devices. For a manually updated list of iTunes versions and downloads (which is far more comprehensive than ) please visit the iTunes page on TheiPhoneWiki.
iTunes 12.8 for macOS and iTunes 12.9 for Windows will be the last available iTunes downloads on and on the IPSW Downloads API.