Mendeley Reference Manager is the newest version of Mendeley, released in 2020 to eventually replace the older Mendeley Desktop. Currently both versions are available and you can choose which one to use, or use them both in parallel. Mendeley Desktop will not be updated, while Mendeley Reference Manager is in continuous development. The library's introduction courses will focus on Mendeley Reference Manager.
If you are using Mendeley Reference Manager you can follow the instructions below. Instructions for Mendeley Desktop are found further down in the menu.