There are few ways how you can install mods to your FS19 server.
After the mods are installed, they still need to be activated. This can be done under the "Home" tab.
To manage the mods on the automatic download feature, simply click the download button to start the download of the desired mod
Mods that are found here are directly managed by Giants, so the version of the mods should always be up to date.
You can upload mods manually to your server by using FS19 Control Panel or FTP client.
To upload mods via FS19 Control Panel, simply navigate to Mods and you will see an option to "Upload mods". Select the .zip archive of the mod on your PC and upload it!
For more informations on how to manually install mods refer to this article: How to manage your Farming Simulator 19 server files using FTP (File transfer protocol)
Disabling and uninstalling Mods
In case you want to disable the mod, navigate to "Home". Find the "Active Mods" section, select the ones you want to disable and press "Deactivate" button.
In case you want to completely remove the Mod that has been taking your Mod space, navigate to "Mods". Simply press the red X near the mod that you want to completely remove from your server.
Keep in mind that the server must be stopped before disabling or uninstalling the mods!