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.

The iPhone 6 is obsolete.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

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.