I am having trouble replacing a part in a part structure?The replace managements tab opens and when i select the part to be replaced and click OK it does not do anything

What you are describing is the use of Variants (Alternates and Substitutes).

If you want to replace a WTPart in a Structure, you will need to use Remove and Insert Existing or Insert New.

There is no Replace functionality available in the Structure tab.  You can use a Replace function in a Change Activity.