how do i reinstall new iPhoto on my mac

Launch Photos from the Applications folder or Dock.

If you specifically need to use iPhoto, put the existing application somewhere Spotlight doesn't index, log into the Mac App Store with an Apple ID previously used to download it, and get it from the Purchases tab. This won't work in Catalina or newer.

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If you need to access your photos in iPhoto, launch Photos with the option key held down. From the dialog, choose your iPhoto Library. Photos will create a new library with your iPhoto media.