For example, say you have a file like Gamemode_Pirate_DM.zip
Now simply rename it to Gamemode_Pirate_Battle.zip and it will show up as "Pirate Battle" in the gamemode name.
You also should change the description.txt to your liking. It will be displayed when a hoster is choosing a gamemode.
You can easily rename the Gamemode file you want to change using FTP for your server hosted by Kalphiter