How do I update iPhoto?

There is no full version of iPhoto that can run on Catalina, because iPhoto needs some 32bit code, you can only get patched, risky versions. There has been no update for iPhoto since 2015, when Apple replaced iPhoto by Photos for Mac and removed iPhoto from the AppStore.. The last system version supporting iPhoto is macOS 10.14.6 Mojave.

You can open the iPhoto Libraries in Photos for Mac to get access to your photos.

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

And: Notes on Migrating an iPhoto Library from iPhoto to Photos for Mac

If you really need to stick with iPhoto because of projects, that need finishing, it would be best to revert your mac to Mojave from a Time Machine backup and make the Catalina upgrade a bit later or install Mojave in a separate partition, just for iPhoto. Revert your Mac to a previous macOS version - Apple Atbalsts

Ignore Iamfunnyninja.

iPhoto will not run in Catalina since it is 32 bit. Photos replaced iPhoto years ago. To use Photos to access your iPhoto Library, Launch Photos with the option key held down. From the resulting dialog, choose your iPhoto Library. Photos will use your iPhoto media to create a new Photos Library. Your iPhoto Library will not be changed.

You can’t ‘update’ iPhotos as an app but you can leave the app and then click it again

Yo, by iPhoto I thought he meant photos app, sorry brother