This is general information based how the heirs perceive their challenges in some cases, but it's important to isolate the issue of the grievance. For example, is your issue that the executor is acting improperly or unfairly, or do you believe the will isn't fair or invalid?
You can save a lot of time and grief by understanding what is and isn't within your control.
If you believe the executor is acting unfairly, you can start by getting a better understanding of the situation. It helps to keep an open mind and remember you could be mistaken. By communicating civilly and requesting the information you're entitled to, you'll be able to see why the results are what they are.
Many people consider what they think the value of the estate might be and their portion, but estates are rarely that simple. For example, an heir who is one of three children, estimates his last-surviving parent's estate to have been about $300,000 and expects $100,000.
In fact, the estate included registered funds which are deemed to have been cashed in on the date of death resulting in considerable income taxes, capital gains tax payable on some other investments, a charitable bequest to the hospital foundation, funeral expenses following an expensive repatriation (parent died out of the country), ongoing bills for utilities, property taxes and the estate wasn't split equally in the will. It's entirely possible the heir might actually receive half or even less of what they expected.
In a case like this, it's important to note the executor has done absolutely nothing wrong. If the heir believes the will was unfair because it should have been split equally, it doesn't matter. Inheritances are gifts, not rights, and they're at the discretion of the testator (except in some jurisdictions where the heir was financially dependent).
In some cases, the opposite of this situation can occur, where the heir was perhaps unaware the parent had been an heir of another estate and their share ends up being far more than they expected.
If communicating with the executor or the information provided fails to yield positive results, heirs can hold them accountable in court. Legal services are recommended though the costs are very unlikely to be paid by the estate unless you win. An estates lawyer can help you assess your chances of being successful.
Challenging the validity of a will can be quite difficult unless there's something technically wrong, such as a missing signature, the testator was unaware of what they were signing (i.e. they were living with dementia), or the testator was unduly influenced (i.e. threatened by a care-giving beneficiary). Legal advice should be sought as this is well beyond the do-it-yourself capability.