To edit your Minecraft server configuration files, open the More menu in your server panel and go to the Configs page. This page allows you to manage supported configuration files directly from the panel.
Using the Configs page is recommended whenever the setting you want is available there, since it is faster and easier than editing files manually in the file manager.
If the configuration you need is not listed on that page, you can then use the Files page for manual changes if necessary.
If you need help with this, please submit a ticket.