You’re on a Mac, I believe, so deleting Dorico 3 works like this:
- Hold down the Opt and Cmd keys while clicking the Applications folder at the right edge of the dock. This will open the Applications folder as a regular folder.
- Drag Dorico 3 from the Applications folder to the Trash. If you really want to clear space, you’ll need to right click on the Trash fan and Empty Trash.
This will not remove your Dorico projects. By default, they are saved in your Documents folder, in a subfolder called Dorico Projects. If you want to back them up, find that folder (via any of the usual methods: the Finder, Spotlight search etc.), copy it, plug your external hard drive in, double-click its icon on the desktop, paste.
The Dorico Projects folder is a perfectly ordinary folder, and it’s just a default location, so it’s quite possible you’ve stored Dorico projects elsewhere. Either way, it doesn’t make any difference to removing Dorico 3.0/3.1 and installing 3.5 - your projects are not stored within the application.