If you plan to update to Dorico 4, I’d suggest starting with that. When you buy your Dorico 4 update, you will receive a Download Access Code (DAC) by email. On the computer where you currently have Dorico 3.5 installed, run Steinberg Download Assistant (download it here if you don’t already have it installed), and sign in with your Steinberg ID. Click the button to enter your DAC, and it will prompt you that you need to update your existing Dorico 3.5 license. Confirm that you want to update.
Now you should see Dorico Pro 4 appear in the My Product Downloads category in Steinberg Download Assistant. Select it, and on the right-hand side, you should choose to install the Dorico 4 Application Installer and the recommended sound content update.
When you come to install on a new computer, it’s easier: download and install Steinberg Download Assistant, run it, sign in with your Steinberg ID, choose “Dorico Pro 4” under My Product Downloads, and on the right-hand side click Install All to automatically download and install everything you need to run the software.