how to import heic files into lightroom 5.7.1 windows 10?

Lightroom 5.x does not support HEIC files. Therefore, you'll need to convert them to JPEG. There are plenty of apps available when you search using Google, although more recent versions of Win 10 should prompt you to visit the Win App store. Below link provides and example of an app that can be used to convert the images

https://www.howtogeek.com/345504/how-to-open-heic-files-on-windows-or-convert-them-to-jpeg/

BTW, if the HEIC files were created on an iPhone / iPad, then best that you change the file type to 'Most Compatible' in devices Settings>Camera>Format. Changing the setting will then mean that your future photos will be JPEG instead of HEIC files.