How do I Switch a datasource/dataset on a existing Rdl file in PowerBi Report Builder?

I Downloaded the PowerBi report Builder, Now i already have a rdl file which i wanted to use but use different dataset/datasource on it.

Now I have created the new datasource in PowerBi Report Builder and i can see that in report Data pane on the left, but the UI still has fields powering from old dataset.

How do i go about that and place some fields from the new dataset in the ame UI??