There are a few ways to change someone elses gamemode. You can either do it in game, through your multicraft control panel or through the server console. The fastest way is in game as you can just type a command and it will change over straight away. This method requires you to have op permissions on the server so make sure you’re logged in as an op and have cheats enabled.
To change a players gamemode you can use the /gamemode command. This can be used to change the game mode of a player on any Minecraft multiplayer server. The first argument is the game mode you want to change to, the second is the name of the player you wish to change it to and the third is the location you want to set it in. This can be a block, a hex or a cuboid selector. The best option is a cuboid selector as it will give you the most accuracy.
If you’re an admin on the server it’s even quicker as you can do it in the server console. This is done by typing /gamemode [creative/survival/spectator] [playername] into the console. This will change the game mode for the player you specified, but anyone who joins afterwards will have to use /gamemode