How do I modify the name of topic in launch file?

edit

Hello, I'm using ROS melodic on Ubuntu 18.04.

I'd like to spawn 4 mobile robots and make them move randomly on gazebo simulation.

I practiced with one robot and also set a node just publishing the topic kinds of geometry_msgs/Twist of cmd_vel.

However, it didn't move at all, because I also had attached the namespace of /robot_0/ to all the topic wrt the spawned robot. (so the name of cmd_vel changed from name of cmd_vel to /**robot_0**/name of cmd_vel.

To discriminate 4 different kinds of cmd_vel topics, I just set like below:

<group ns="robot_0">
<param name="tf_prefix" value="robot_0" />
 <include file="$(find turtlebot_gazebo)/launch/includes/$(arg base)_object.launch.xml">
   <!-- <arg name="init_pose" value="-x 0.5 -y 0.5 -z 0" /> -->
   <arg name="init_pose" value="-x 5.5 -y 5.5 -z 0" />
   <arg name="base" value="$(arg base)"/>
   <arg name="stacks" value="$(arg stacks)"/>
   <arg name="3d_sensor" value="$(arg 3d_sensor)"/>
   <arg name="robot_name"  value="robot_0" />
   <arg name="color" value="red" />
  <include>
</group>

But in the case of I made the lines wrt namespace be comment line like this,

<!-- <group ns="robot_0"> -->
<param name="tf_prefix" value="robot_0" />
<include file="$(find turtlebot_gazebo)/launch/includes/$(arg base)_object.launch.xml">
  <!-- <arg name="init_pose" value="-x 0.5 -y 0.5 -z 0" /> -->
  <arg name="init_pose" value="-x 5.5 -y 5.5 -z 0" />
  <arg name="base" value="$(arg base)"/>
  <arg name="stacks" value="$(arg stacks)"/>
  <arg name="3d_sensor" value="$(arg 3d_sensor)"/>
  <arg name="robot_name"  value="robot_0" />
  <arg name="color" value="red" />
</include>
<!-- </group> -->

It moved.

But I couldn't get the topic discriminated against following the namespace which means I couldn't distinguish which cmd_veltopic is from which robot.

What I want to do is, 1. To make the robot move. 2. To discriminate the topics following the 4 spawned robots.

If anyone can answer or give me advice, would you please give me help?

Thanks in advance. :)