how to update from iOs 12.5 to iOs 13.4

You can't. An iPhone on any version of 12.5 will always be a 5s, 6, or 6 Plus, none of which are capable of 13 or above.

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Hey there,

Please let us know what is your iPhone model?

If you have iPhone 6/6plus , this device can’t be able to get update anymore, the last iOS version for this device is iOS 12.5.7!