You can change to a specific Fabric version in case certain mods require it. The following steps will change both the Minecraft and Fabric version.
1. Navigate to Fabric's official website and download the server jar file for the appropriate version.
2. Open the File Manager
2. Navigate to /Minecraft/
and upload the Fabric server jar file.
3. Go back a directory to /
and right click docker-compose.yml
and click Edit
4. Add the line FABRIC_LAUNCHER: <fabriclauncherfilename>
5. Save Config & Restart the Basic Server Settings