1
First Step
Log in to your Wepwawet Control Panel and select your Server
2
Second Step
Stop your server.
3
Third Step
Under the General tab, Click on Files, Then select world, world_the_nether and world_the_end and click on the Delete button at the top.
4
Fourth Step
After that, Click on the server.properties file and go to the line level-seed=[seed] and change [seed] to your prefered seed that you’d like to make your world.
5
Fifth Step
Click on Save Content or press CTRL+S
6
Sixth Step
Under General tab, Click on Console and Start the server.
There you go your server now has a new world with your new custom seed!