Colin, you have to right click on the part you want to replace in the feature tree. Look for replace component, you may have to hit the arrows at the bottom of the menu that pops up to see the replace component icon. Browse to where the new file is.
Another way to replace a component is to hit file, then about half way down is the replace icon. I have always used the above method because it fills in the part you want to replace in the top box.
Also in the top right is the search box and you can type in a command and then hit the eye glasses and it will show you where a command is located.