How to download iOS 15.17?

Did you try to follow the steps mentioned in this article?

Update your iPhone or iPad - Apple Support

Or, if you are using a computer or Mac:

Update your iPhone, iPad, or iPod touch with iTunes - Apple Support

Update your iPhone, iPad, or iPod touch with the Finder - Apple Support

Help me with my phone

Click on the blue links above to find out more about how to upgrade your device.

Depending on your built in iPhone hardware, you might not be alle to install the latest iOS version:

iOS 13 and iPadOS are compatible with these devices - Apple Support

iOS 14 and iPadOS are compatible with these devices - Apple Support

iOS 15 and iPadOS 15 are compatible with these devices - Apple Support

I cannot download some apps on my phone

What phone? What happens when you try? What error message do you get?