Prefix used to be set with a command, but now, it is in our dashboard.
As our new dashboard coming out, we are trying to switch everything database related. Our first move was the prefix. Now you could change prefix on our dashboard in less than 1 minute.
Step
1. Go to our dashboard(assuming you know how to login)
2. On the top navigation bar, select Prefix.
3. Choose you desire server
4. Enter your prefix
5. The prefix should be set.