The typical action to challenge an inventory and appraisal filed by a personal representative or a court-appointed attorney is termed an, "objection." You would file the objection with the court, serve it on all other interested parties (beneficiaries, personal representative/executrix,court-appointed counsel, and anyone else who has file a request for special notice with the court), and provide your evidence and argument as to why the inventory and appraisal is in error.
There won't be a "form" that you can file -- so, if you don't know how to prepare a motion/objection pleading, then you would either have to spend a lot of time at a public or university law library, or you will need a lawyer to draft the pleadings, and potentially represent you in court.
Probate courts are somewhat more lenient with unrepresented parties, because of the nature of the proceeding. However, if there is a lot of money at stake, then in my opinion, you would be far better served to hire a lawyer to represent you, because once a lawyer is involved, the court will pay greater attention to the dispute. Judges usually give more credence to lawyer arguments, because, after all, judges are lawyers, too, so they tend to believe the lawyer's arguments in favor of a layperson.
And, if you're representing yourself, and your adversary is a lawyer, then that lawyer will get most of the attention -- which, may not be fair -- but, it's reality, so you need to be prepared to deal with that reality.
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