The Scorch download page contains the latest installer. However, in order to download to correct installer, you will actually have to use Chrome to access the page—otherwise, you won’t get the correct installer.
Sam Butler at Avid provided some more helpful information:
If you have problems installing this update, first uninstall any previous version of Scorch you have by going to Start, Control Panel and then Programs and Features. Uninstall Sibelius Scorch from there and then you should be able to run the new installer.
A note for Mac users: we are continuing to work on a solution to run Scorch in the latest version of Firefox. For now, please continue to install the ESR release of Firefox.
We will post news on any further developments as we have it.
We further learned on the Sibelius forum that Score Exchange (the successor to SibeliusMusic.com) has lifted the block on viewing scores on Chrome on Windows, and that Avid is working on an update to Scorch for iPad.