Download iTunes 12.6.3 for Mac or Windows from Apple support here.You can download iTunes 12.6.3 from the Apple support page, or using the direct download links below which point to the files directly on Apple servers:

Download iTunes 12.6.3 with iOS App Store Support

The ease of installation prevents the need to downgrade iTunes 12.7 or fiddle with the somewhat hidden iTunes 12.7 method of transferring apps and ringtones to an iPhone or iPad. Users who would like to have the iOS App Store functionality in iTunes on their computers again should download and install the alternative iTunes release. ITunes 12.6.3 is available to download for Mac and Windows users, and can be easily installed over iTunes 12.7 to regain the native iOS App Store functionality for improved iPhone and iPad app management. Apple apparently released iTunes 12.6.3 as an alternative version because “certain business partners might still need to use iTunes to install apps.” But even if you are not a “business partner” you can still download and install iTunes 12.6.3 and use the version of iTunes to manage apps with an iPhone or iPad connected through a Mac or Windows PC.

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