ANSWER:
A codicil is a supplementary Will i.e. in addition to the main Will. By itself it is a valid Will. When properly completed a codicil has all the formalities and validities of a complete Will. The codicil must be signed by the testator in the presence of 2 witnesses in the same manner as a Will.
Example of a codicil :
This is my codicil and is supplementary to my Will dated the 10 day of June 1997.Paragraph 5 of my Will is revoked and replaced by the following paragraph.
So a codicil is a testamentary document altering an existing Will. If possible, the codicil should be attached to the Will. But it is better to make a new Will if there are substantial alterations to be made.