if Apple is the developer how can we get update for iPhoto?

Here are a few hints on how to upgrade from iPhoto to Photos for Mac: Updating from iPhoto to Photos for macOS – Apple Support

Photos for Mac will allow you to access the photos in your Photos Library, and then you can decide which application you want to use to manage your Photos.

Here is the online user guide for Photos: Photos User Guide for Mac - Apple Support

Here are some additional user tips from Photos for Mac  User Tips that will help you in your transition from iPhoto to Photos:

How to prepare the Photo Libraries before upgrading to Catalina - Apple Community ➙ read first 

External Editors in Photos Are Here

Photo Editing Extensions for Photos for Mac

Post 9/30/2018 Printing Options for Photos - Apple Community

Ordering Books and Calendars on a Mac that cannot be updated past El Capitan 

Use Project Extensions to order print products in Photos after September 30, 2018 - Apple Support

Create and order print products in Photos with Project Extensions - Apple Support

For the OP... here is a user tip provided by léonie: How to prepare the Photo Libraries before upgrading to Catalina - Apple Community

More here: https://discussions.apple.com/profile/léonie/subscriptions?page=1&content=filterContent&activity=filterAuthored&type=filterUserTips

Use Photos, which is included with macOS Catalina - Apple.

You don't. iPhoto was discontinued several years ago. It has been replaced by Photos.