An amendment to a will is called a "codicil." Writing a codicil does not require the help of a lawyer in any part of the United States, but a codicil must be written with the same formalities as a will. The basic rules are that the person writing the codicil must have the intent that it become his will, and it must be signed by the writer and two adult witnesses.
Display your mental clarity and understanding of what you are doing as you write the codicil. Use language such as, "I, [name], on this day [date], in [location], do hereby amend my last will and testimony as follows." Then specify the changes you want to make to your will. For a codicil to take effect, the writer, who is the testator, must have sufficient mental clarity and understanding to know what he is doing as well as intent to make a codicil right then.
Sign the codicil, or tell someone to sign it for you. Most states require the testator to sign the codicil or for the testator to direct another person in his presence to sign the codicil. Some states do not require a signature if the codicil is signed by witnesses, but even then it is good practice to sign the codicil; signatures reinforce the presumption of intent. The signature need not be the testator's full legal name. An informal name or initials suffice.
Tell your two adult witnesses to sign the codicil immediately. In general, a codicil must be signed by two adult witnesses. A large minority of states do not require witnesses if all material portions of the codicil are handwritten. Nonetheless, it is good practice to have at least two adult witnesses because their presence and signatures create the presumption that the codicil was duly executed; they could testify later that the testator was of sound mind when the codicil was written. Each witness for a codicil must witness the testator's acknowledgment of his signature on the codicil. It is enough for the testator to state, "This is a codicil to my will. This is my signature. Please sign your name here."