I need ios13 for some app. What can I do? My iOS is 12.5

An unit running any version of 12.5 isn't capable of 13 or above. Unless you've previously downloaded that application to a newer iPhone and the developer has left a compatible version of it available, you can't get it on there.

(197967)

The iPhone 6 will not update to iOS 13. If you need iOS 13 or higher, you will need to purchase a newer iPhone. The iPhone 6 will not support anything higher than you already have.

Update your device:

https://support.apple.com/en-us/HT204204