How to update ios 12.5.7 to ios 15

An unit running 12.5.7 isn't capable of 13 or above; they require at least 2GB of RAM.

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My iPhone iOS version is 12.5.7 but I can’t download Facebook and Snapchat why

Most apps now need iOS 13 or newer to work. You need a newer phone.

How to update ios 12.5.7 to ios 15

This is not possible.

You need a new phone that is capable of running at least iOS 16. iOS 17 would be much better.