A seed is only used when a brand new world is made. So you set the seed, give the world a new name, and the server makes it for you.
Make a world from a seed
- Stop your server.
- Open Properties, and search for
level-seed. Paste your seed, then click Save. - Open Startup. Under Choose World, type a new name for your world, like
seed world. - Start your server. It creates your new world from the seed.
Your old world isn't deleted, so you can switch back to it any time.
Go back to your old world
Change Choose World in Startup back to its name, and restart your server.