How do I update iPhoto on catalina

There is no iPhoto update for Catalina. Apple stopped the development of iPhoto. The last update has been iPhoto 9.6.1 for macOS 10.103 Mojave, five years ago. iPhoto needs some 32 bit code, that cannot run on Catalina.

You can open your iPhoto Libraries in the Photos.app. It is the replacement for iPhoto to be used on the new Macs and recent system versions and supports modern hardware and iCloud Photos Library.

See: Updating from iPhoto to Photos for macOS – Apple Support

You're welcome 😊