Manage the Minecraft server, then type “Seed” into AMPs top right search box to find the setting. You can use the search box to find any setting within a given AMP instance.
Restarting the server won’t do anything. The Seed is used to generate the world, its not used beyond that point.
You’ll need to either delete/rename the old world file in File Manager, or change the Level Name setting so you generate a new world with your chosen seed.