Please Follow The Guide located:
https://docs.google.com/document/d/14eB_P9IYjR_xuG1pSW5J7fR73fVK373Agi6-A-tadwQ/edit?usp=sharing
Download the Sibelius 8.5.1 from the below links:
 .zip for Windows:
https://drive.google.com/file/d/12hXiziGvtCCKAYGUTNrNv11V1xqC9jVL/view?usp=share_link
.DMG for Mac:
https://drive.google.com/file/d/12qGRkYU9FgTbX3uPvR0guYX9zl29bmCo/view?usp=sharing