In the change node selett the jsonata
option then use the expression
payload[$$.DropBox]
and see if that works.
here is a simple inject
->change
->debug
flow using msg.array as the array and msg.topic as the index value:
[{"id":"2517e138.caf176","type":"debug","z":"7abfc37c.974ed4","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"true","targetType":"full","statusVal":"","statusType":"auto","x":190,"y":180,"wires":[]},{"id":"65582247.f6b41c","type":"inject","z":"7abfc37c.974ed4","name":"","props":[{"p":"array","v":"['cat','dog','frog']","vt":"jsonata"},{"p":"topic","v":"2","vt":"num"},{"p":"payload"}],"repeat":"","crontab":"","once":true,"onceDelay":0.1,"topic":"","payload":"","payloadType":"date","x":210,"y":60,"wires":[["d24362d7.3109a8"]]},{"id":"d24362d7.3109a8","type":"change","z":"7abfc37c.974ed4","name":"","rules":[{"t":"set","p":"payload","pt":"msg","to":"array[$$.topic]","tot":"jsonata"}],"action":"","property":"","from":"","to":"","reg":false,"x":220,"y":120,"wires":[["2517e138.caf176"]]}]