how do I update iPhoto to 64 bit?

The Photos.app can open the Photos Library or an iPhoto Library, but the photos are not stored in the Photos.app. Photos is just the program to manage the photos. The libraries are containing the photos and videos. Look in your Pictures folder for this icon - Photo libraries are usually showing a fan of three pictures:

For starters there id Photos included with your upgrade if that meets your needs.

all your iPhotos are in the photo file...

You can't. Launch Photos from the Applications folder or Dock and use it instead.

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You purchase a different photo app to replace it.

What app? and how do you replace photos that you cannot find?

Where are the photos I had in iPhoto?

The iPhotos library should be in ~/Pictures unless you moved it.

If you start Photos with the Option key depress it will allow you to search for the iPhoto library.

I want to update my iPhoto and toast tiatim to work on my Mac 10-15.1

Apple stopped the development of iPhoto in 2015. There have not been any updates for iPhoto since then. it is no longer sold from the App Store. Photos for Mac replaces it.

No more iPhoto in Catalina. You can go back to the older OSX, or make a partition and use parallels (Virtual machine) to get access?