To make changes to a will, my suggestion, and I believe most attorneys would agree, is that you need a new will.
Any changes to a will must be done with the same formalities of the original will.
This means witnesses, at least two under New York State law, and properly done.
To me that means it is done under the supervision of an attorney, like myself, sitting at a table with my staff as witnesses, notarized documents, and all the like.