The following article will cover the steps needed to change your server name on your Arma 3 Server Hosted By Host Havoc.
1. Log into your game panel.
2. Stop your service if it is running.
3. Click on 
in your game panel and then click on the 
for the server.cfg file.
4. Locate the //password = "ServerPassword"; section and remove the two // at the beginning of this line. Now change ServerPassword to whatever you'd like and click on 