You just need to run the Fabric installer again and pick loader 0.18.3 (or newer) for your Minecraft version.
- Download the Fabric installer from the official site.
- Open it, choose your Minecraft version, set the Loader Version to 0.18.3 (or any newer drop-down option), and install.
- Start Minecraft once so the new loader version is written to the instance.
- Double-check that the modpack/profile points to that updated Fabric loader version (some launchers cache the old one).
That will satisfy FancyMenu 3.8.3’s requirement.