It's easy. Click on the structure to highlight it, then right-click for the pop-up menu. On that menu, there's a command to 'Swap Part...'. It brings up the part catalogue to pick the next part to change it to. Be aware that this also updates the part description, so you may want to check that after you swap the part.
If the pop-up menu doesn't show a 'Structure Properties...' or 'Swap Part...', there must be 2 or more items highlighted. Hit ESC a couple of times and try again.
HP Z600 8 Gig RAM
Civil 3D 2011