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.

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.

You can not update that phone any further. It is obsolete.

You will need a new phone.