How to update software 12.5.2 to recent software update in iphone 6

If an app needs iOS 13 or higher, you will need to get a newer phone if you want to use them. An iPhone 6 will never update to iOS 13 or 14.

You don't with an iPhone 6. 12.5.2 is the highest it will go.

If apps you are trying to use say you need a higher iOS version, you will need to buy a newer device if you want to run those apps or features.

The iPhone 6 is obsolete. You will need a new phone.