Qt 5.12 LTS will receive multiple patch releases throughout the coming ~3 years and we recommend all active developed projects to migrate to Qt 5.12 LTS. Qt 5.9 LTS is currently in 'Strict' phase and receives only the selected important bug and security fixes, while Qt 5.12 LTS is currently receiving all the bug fixes. With Qt 5.6 Support ending in March 2019 all active projects still using Qt 5.6 LTS should now migrate to a later version of Qt.
Qt 5.12.1 can be updated to using the maintenance tool of the online installer. For new installations, please download latest online installer from Qt Account portal or from qt.io Download page. Offline packages are available for commercial users in the Qt Account portal and at the qt.io Download page for open-source users. You can also try out the Commercial evaluation option from the qt.io Download page.