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If you want to use Dashing all you need to do is following the Dashing specific instructions: https://index.ros.org/doc/ros2/Installation/Dashing/

How should I only update some certain package such as only update RVIZ?

If you are asking how to use a newer version of rviz (e.g. the Eloquent one) with Dashing then the answer is: don't.

There is no guarantee that this will work. Often new ROS distributions come with changes / additions and if rviz in Eloquent has been updated for them it won't work with Dashing anymore.