Now I upgrade my AWstats from 6.5 to 6.6. All safe.
1) Backup all files in your wwwroot. Just in case.
2) Download the awstats-6.6.zip, and decompress it.
3) Copy the files in wwwroot and replace the old files.
4) Done.
The configuration file is same. I don’t need to change anything, just copy and paste.