There are two ways you can install Fabric on your server, via our version install or manually yourself.
To download Fabric via our version installer, select Fabric from the drop down, then select the version you need.
Once its downloaded restart your server, and you're all set.
There are number of reasons why you might want to install Fabric manually. Once reason could be you're looking for a set Fabric version that isn't on our version installer.
First, download Fabric from here: https://fabricmc.net/use/server/ (opens in a new tab)
Use the drop downs to select the version you need.
Upload the jar it downloads via the file manager.
Head to the Overview page and set the jar name. It might be too long, so you may need to rename the jar first.
Restart/start your server. You're all set Fabric is now loaded on your server