Given that you will be replacing the tweeter I suggest you turn the speaker upside down, let the loose item fall down and retrieve it via the tweeter cutout. You may have to use a flashlight to see the item's location.
Hopefully the replacement tweeter will match the tone etc of the other tweeter but it may be that you end up having to replace the working tweeter as well to balance things out.
Good luck.
[added]. Are you sure the tweeter needs replacing? Maybe a cap on the crossover has drifted?
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