Make sure your server has a full backup.

In order to reinstall the server, you can use this menu in the panel of your server

Go to "General settings" and find this window

Check the box if you want to delete all files. By default, reinstalling the server keeps some custom files.

Then click "Yes, reinstall server"

How to change the game or choose a different version of the server

Go to "General settings" and find this window

Check the box if you want to delete all files. By default, reinstalling the server keeps some custom files.

Then click "Yes, switch"