Your world seed determines how your world is generated.
If you change the seed, it will only impact the chunks that are created after the edit. To make the whole world use a new seed, you need to delete your world folder, but keep in mind that this will wipe out all your progress and start everything over. Take a backup if you are not sure what you are doing.
Changing your Server Seed:
1. Log into the Pterodactyl panel and open your server
2. Head to the File Manager
3. Navigate to server.properties
4. Change level-seed=random_seed to level-seed=desired_seed
5. Head to the console page and restart your server
