To change the gamemode, go to Configuration and then select Command Line Manager.
There you will see a box called Gamemode, in which you can input the name of the gamemode that you wish to start your server in.
Once you've entered the gamemode that you wish to start the server in, scroll down to the bottom of the page and click the button called `Update Startup Parameters`.
For the changes to take effect, all you need to do now is restart your service by clicking the Restart button on the Console page.
Note: If you don't know what to enter in the box, then just look at the name of the gamemode's folder. For example, DarkRP's folder is called `darkrp` and TTT's folder is called `terrortown`.