A codicil is an instrument made in relation to a Will, explaining, altering, or adding to its dispositions and is deemed to be a part of the Will. A codicil has to be executed and attested like a Will. A codicil is similar to a Will and is governed by the same rules as a Will.
A change can be in the form of addition, deletion, or substitution of some clause or name or property or date, etc, with a new one, or any other type of change that you wish to do in your Will. For this, you cannot make these changes directly in the Will document, and if any alterations are seen in the Will document, it becomes void.