The last will and testament a person might execute is not necessarily the final word on how that person's estate will be distributed when the will-maker dies. If you are the will-maker, you are free to change your will or revoke it entirely, as long as you are mentally competent. However, simply altering provisions in your will by marking over them may not accomplish your intent.
Altering a Will
Changes made to a will by a third party are referred to as spoliation or mutilation, which is illegal, without your consent. If you try to edit your own will, it is called alteration. Even if it is your own will, your desired changes are enforceable only if you make them according to state law.
Modifying a Will
A will is a legal document, so it isn't wise or effective to simply edit it with a red pencil. Crossing off provisions or scribbling in new provisions at the bottom of the document will not create valid bequests, and these may invalidate the entire document, depending on the rules of your state. Any modifications for provisions in wills must be made in the same way you executed your initial will -- generally, this is done with witnesses who sign after you. It is often as easy to revoke your will and draft a new one than to add codicils altering the provisions of the will.