You can name any competent adult as the executor of your will. People commonly name a family member as the executor. This same person is also often a beneficiary of the will. This is allowed in most states as long as the person's interest in the estate is not in dispute. If you want to rewrite your will to name one of its beneficiaries as the executor, you can do so at any time.
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Rewrite your old will, replacing the current executor's name and address with the beneficiary's name and address.
State in the new will that you are revoking the old will. If the only thing you want to do is change the executor, you do not need to make any other changes. If you want to change any bequests, make those changes as well. For example, if you want the new executor not to be a beneficiary at all, name new beneficiaries for the property you were bequeathing to her.
Execute the new will in the same way that you executed the old will. In most states this requires you to sign the will in front of two witnesses.