An executor of an estate is the person who is legally responsible for carrying out the will and liquidating the estate of a decedent (someone who is deceased). An executor is usually named in a will or can be appointed in court if no executor was named or there are legal challenges to the will (usually called an administrator if not named in the will). An executor can voluntarily choose to give up his duties, but can only be removed for incapacity, ineligibility (a felon, conflict of interest, lack of a required bond) or clear misconduct.
Document the circumstances that led you to want to change the executor of an estate of which you are a beneficiary. A detailed record of the problems and/or alleged malfeasances of the executor is essential to convince a judge to change an executor. The evidence must be compelling; keep in mind that judges are unlikely to replace an executor simply because a beneficiary feels he is not getting his "fair share."
Contact an attorney experienced in wills and probate and show him the documentation you have gathered. If your evidence offers sufficient grounds to show the incapacity, ineligibility or malfeasance of the executor, your attorney can file a petition with the judge in probate court to have him replaced.
Appear in court with your attorney and present your evidence to the judge. It is usually a relatively short proceeding, with the judge typically taking the documents offered by both sides for consideration before rendering a decision (sometimes days or weeks later).
Often the best (and easiest) way to replace an executor is for you and your attorney to meet with him and present your evidence and request he voluntarily resign his position. That means both sides can avoid legal expenses and save time, and the executor avoids damage to his reputation.