Eco has excellent mod support and a large community that supports them. Using this guide you'll be able to easily install any mod that is compatible with dedicated servers.
Be sure to also read any additional or different installation instructions for the mod on the mod page. Not all mods will have the same installation steps, but this guide goes through the general way to install the vast majority of mods.
1. Download the mod from Eco mod.io
2. Open the File Manager
3. Drag and Drop the downloaded zip to the server and extract it
4. Restart the server