How to Update iPhone 6LS to ios12.5.3

I've never heard of an iPhone 6LS. Did you mean iPhone 6s?

You can update an iPhone 6s or 6s Plus to the latest version of iOS 15 (currently iOS 15.8.2). The iPhone 6s and 6s Plus cannot run, and will never run, iOS 16 and higher.

About iOS 15 Updates - Apple Support

Update your iPhone or iPad - Apple Support

Updating iOS: common issues and solutions - Apple Community

If what you have is actually an iPhone 6 or 6 Plus (no 's'), you can only update those phones to the latest version of iOS 12 (currently iOS 12.5.7). There are no updates to iOS 13 or higher for the iPhone 6 and 6 Plus.