How can I upgrade my iOS from 12.3 to 13.0

You cannot. This is because you have an iPhone 6 which can only be updated up to iOS version 12.x.x. You will need an iPhone 6s or newer to update to the current version which is iOS version 14.x.

Axel F.

kleatch Said:

"How can I upgrade the iOS from 12.3 to 13.0"

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Upgrading is not an Option:

Your iPhone model is incompatible with iOS13, then because you iPhone does not meet the hardware requirements for this iOS version(s).

An iPhone 6 isn't capable of 13 or above. To update it to 12.5.4, follow the instructions in the Related Article above.

(201867)