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

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.

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.

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Update your device:

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