To start editing, click on the the railing whose top rail you wish to edit. In the properties panel that opens on the left, click on the Rails drop-down menu. Next, click on the Top Rail drop-down menu and click on the toggle button to switch on its visibility.

Go to the Placement menu in the left panel and enter the desired value into the Elevation field.

Go to the Placement menu in the left panel and enter the desired value into the Offset field to offset the rail outwards. To offset the top rail inwards, type in a negative value.

Click on the Edit Profile option in the left panel, and you will enter the Sketcher mode, and sketch the closed curve/polyline intended to make up the cross-section profile for the top rail. Click on Apply to save changes.