You've done the responsible thing and written your will, explaining how your money, property and other assets should be distributed after your death. Trouble is, things have changed. Maybe you have new assets. Maybe you want to change the terms or beneficiaries of your will, but you don't want to write an entirely new document. Add an addendum to your will. It will be as valid as the original will and, if it creates a change, will supersede the original terms of the will.
Contact your lawyer. Explain that you want to add an addendum -- also called a codicil -- to your will.
Write your codicil, beginning it as "Codicil to the Will of (your name)." State the city and county in which you live and that you are adding this codicil to your will. Indicate that you're of sound mind and body and are creating the codicil of your own free will.
Indicate the creation date of the will to which you're adding the codicil. State something along the lines of, "This is the third codicil to my will, which is dated February 2, 2009."
Specify the changes being made through the codicil. If you're revoking part of your will, directly state which paragraph you're taking back. If you're adding to your will, include the details of the addition.
Give the codicil to your lawyer. Ask him to add it to your will. Sign and date the codicil. Give a copy to each person who has a copy of your will.
While most states allow you to create a will without a lawyer, it is best to have a lawyer create and sign your will. Will law is complex and lawyer-created laws will hold up better in court.