Select a component-instance.
Right-click Context-menu > Edit Component
You are then inside the component and can select/delete/move/change its geometry.
So in your case - select and erase/delete the vertical parts of the box forming the rectangular post.
Draw a Circle centered on the base’s left over square, PushPull it up to the required height - given by the left over top square.
Erase the unwanted top/bottom squares.
Exit the Edit - either though the Context-menu or by being in Select and picking outside of the component’s ‘bounds’.
All columns using that definition should now appear updated.
If you have more than one component definition either repeat the editing on those that that ‘different’, or if they are the same as one you’ve already adjusted you can select the wrong ones and use the Component Browser to select the required version and use Replace Selection fly-out option - then the select items are changed to use the same definition…