Drop target assemblies... groan. Probably the worst job of them all. First rule is to NEVER take it apart unless you really really have to... and I mean really really.
I have some looong screws that I use.. take the old ones out ONE at a time and replace with the longer ones.. if you take them all out and it falls apart you will regret it.
Longer screws allow you to loosen  the plates enough to swap over the drop target and change the spring if necessary... plates are loose but not separated.. longer screws.. remember.. longer screws.. don't let it fall apart