This is some kind of common problems since I googled for answers. I believe this problem is some thing to do with web server file ownership.
In my case, I am using web server “Nginx”, which has user and group under “www-data”. All I have to do is to change the group ownership to “www-data” for files in specified WordPress installation folder.
This linux command should solve the issue:
“chgrp -R www-data wordpress_installation_folder/”
Option “-R” is to recursively update the group change over files including all sub-folders.
Then we check “ls -al” to make sure the file permission in group is under “www-data”.
After that, we refresh the WordPress update link to see WordPress updating in progress.