2. At the left-hand side of the control panel, you can see the Chat module, Console module, etc. Choose "Files", then choose "Config Files".
3. Select "Server Settings" from your list of available configuration files. Depending on your server type, you may have several options on this screen. However, "Server Settings" will always be at the very top.
4. Scroll down through the list of configuration options until you see "Game Mode". In the dropdown box beside it, you'll be able to click it and choose from Survival, Creative, and Adventure mode.
5. After selecting your desired game mode, scroll down to the bottom of the page and click "Save".
You may now start up the server again, and all new players will join with the game mode you've just selected.
If players were previously on your server using a different game mode, they will not join with the server's default game mode unless you also change the Force Game Mode configuration option to Enabled. This option is also located in your Server Settings. You may change it at the same time as changing the server's game mode.