Why can’t I update me iOS software version to iOS 13

The iPhone 6 does not have the required hardware to update to 13 or higher. The highest possible for that model is iOS 12.5.5.

iOS 13 and higher requires 2GB RAM or more and a 64bit platform. iPhone 6/6+/5s has 1GB RAM

An iPhone 6 doesn't have enough RAM for 13 or above; this isn't upgradable.

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