This should take you to the Mendeley landing page. From here, you can click on the link to Download Mendeley Reference Manager:
The Mendeley website will detect your operating system. Click on the button Download now for [Windows or macOS or Linux]
Your download should begin straight away. Follow the usual process for installing software on your computer.
Once Mendeley Reference Manager is installed on your computer, you can open the programme and sign-in with your Mendeley (Elsevier) username and password.