Download macOS High Sierra 10.13.5 Update
The macOS High Sierra 10.13.5 update improves the stability, performance, and security of your Mac and is recommended for all users.
This update adds support for Messages in iCloud, which lets you store messages with their attachments in iCloud and free up space on your Mac. To enable Messages in iCloud, go to Preferences in Messages, click Accounts, then select “Enable Messages in iCloud”.
For more detailed information about this update, please visit: https://support.apple.com/kb/HT208723