A codicil (from the Latin codicillus, meaning “a small writing tablet”) is a legal document in which you can make minor changes to your existing will. That said, rewriting your whole will can sometimes be a better option – particularly if the changes are substantial or complex, or if you already have a codicil (multiple codicils can be confusing!).
Remember that it must comply with exactly the same signing and witnessing rules as a will to be valid. And, on a practical level, be sure to attach the codicil to your will as an addendum, and to tell everyone who needs to know about it, especially your executor.