Sadly, I am not sure you can update your phone. I think this topic was discussed last August here: https://discussions.apple.com/thread/251647829?page=1. Have a look and see if it answers your question. Otherwise, go to 'Get Support' at the top of the page and ask Apple Support if it can be done.
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2 replies
You can not update that phone any further. It is obsolete.
You will need a new phone.
I using iPhone 6 Plus.i have updated iOS 12.5.3 version. But required iOS 13 or above version.but again I was trying to updatiie but not done. Please help me how to update iOS 13 version.