You may have an existing Will which you are largely happy with, but find that you now wish to make one or two minor tweaks to it. In this situation, you can update your Will without completely rewriting it, by making a Codicil. The Codicil is usually a short document which will refer to your pre-existing Will, and will record that you confirm the terms of your Will, apart from the changes now being made to it by the Codicil.
Examples of provisions typically included in a Codicil might be, for instance, making a change to your executors, or perhaps adding a legacy to grandchildren. Other information that might be included in a Codicil could be updated addresses for your executors or beneficiaries, perhaps recording that someone has predeceased you, or removing a clause from your Will following a change in the law which has made it unnecessary. None of these latter changes are essential, but they are certainly helpful to your executors. If the main provisions of your Will need to be rewritten, however, it is always best to make a new Will.
Depending on the changes that you want to make to your Will, you may also feel that it would be more tactful to make an entirely new Will. Your Will and Codicil will become public documents after your death, and if for example you were removing a gift to somebody made in your Will via a Codicil, they would then potentially be able to see what had happened, which could be contentious. If you made a new Will which did not include them, they would not be able to see the sequence of events.
The signing formalities for a Codicil are exactly the same as those for a Will. Your signature will need to be witnessed by two independent, adult witnesses, in order for the Codicil to be valid.
Once signed, your Codicil should then be stored with your original Will, to ensure that it is not overlooked by your executors. It would also be good practice to mention to your executors that you have made a Codicil.