WordPress 4.9.1 is available! Please update now.

You can update to WordPress 4.9.1 automatically:

I ran the update and got his message:

Unpacking the update…

Verifying the unpacked files…

Preparing to install the latest version…

The update cannot be installed because we will be unable to copy some files.

When I googled the error I discovered there are multiple reasons for the failure.

WordPress does not always know and report what has caused the issue.

In my case I had run out of disk space.

• Fix: Insufficent Disk Space

I had run out of disk space, even though the failure message did not say that.

To work out whether this is the cause, login to cPanel (hosting control panel) and check Disk Usage.

WordPress requires space in which to download the update files, unpack (extract) and upload the files.  Each update would be different but at least 100Mb

• Access files (FTP or File Manager) and delete files to increase available space.  Possible files that could be removed: error_log, old files, tmp files, anything that’s no longer used.  Make a back-up just case.
• Contact your hosting provider and request a disk space allowance

Once the space is availble, run the update again and voila,

Welcome to WordPress 4.9.1

Thank you for updating to the latest version! WordPress 4.9.1 will smooth your design workflow and keep you safe from coding errors.