(law) An addition to a will, that changes, explains, revokes, or adds provisions.
A testamentary document that adds to, subtracts from, qualifies, modifies, revokes, or otherwise alters or explains an existing will. To be valid, it must be executed with the same formalities as a will.
Origin of codicil
Middle English from Old French codicillefrom Latin cōdicillusdiminutive ofcōdexcōdic-codexcodex
American Heritage Dictionary of the English Language, 5th Edition
Borrowing from Middle French codicille, from Latincōdicillus, diminutive of cōdex. See code
Codicil Sentence Examples
By his will, made in 1852, with a codicil dated February 1859, his property, with the exception of some small bequests, was devised to the above-mentioned institution at Berlin.
A change or addition to a will is called a codicil.
I suggest you see the Solicitor who originally drew up the will to make the codicil for you.