how to update IOS 12 to 13?

An iPhone 6 can't be updated beyond 12.5.7, it doesn't have the hardware needed to run anything higher.

If you need a higher iOS version, you will have to purchase a newer phone.

To update to 12.5.7 -

Go to Settings > General > Software Update.

This link will show you how to update your iPhone, iPad, or iPod touch using WiFi -

Update your iPhone or iPad - Apple Support

This link will show you how to update using your computer -

Update iOS on iPhone - Apple Support

"how to update IOS 12 to 13?: I cannot download a new app because need to update IOS. Please help me with this matter. Thank you."

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Updating an iPhone 6 Past iOS 12:

A. You can't.  The iPhone 6 will only install to iOS 12.5.7. See here: Update your iPhone or iPad Software if you're Experiencing Issues with Location, Date, and Time - Apple Support. And, for the note, even if you could update, only the latest version will install: iOS 16.

So, what to Try:

B. Update Using a Computer:

Have a Computer Handy? If so, try using a computer to update. Go here: Update your iPhone Using a Computer - User Tip. If any updates are availabe, they will install, sure enough.