I can't install macOS Sierra on my MacBook Air

I'm having trouble installing macOS Sierra to my MacBook Air (Mid-2012). Whenever I click restart, and the install goes installing the version, it just says an internal error occurred when the download bar is at 29 minutes remaining. I tried to remove files that take up space, waited a few weeks after the version was released, but still it keeps saying an internal error occurred. Is there a solution to fix this and I don't want to lose data (like messages, apps and files)