What would I have to do if I want to delete adult child from will and add a adult child previously deleted? Can I use codicil or will I have to do new
You could do this by codicil but frankly it would be better to draft a new will . For one thing it would lessen the chances of some type of mistake or misinterpretation; when possible it's best to have one legal instrument instead of two. Secondly, if you want to disinherit a child you should really do so by inserting a disinheritance clause into the will itself. This insures that your intent to disinherit an heir is unequivocal. The clause acknowledges the relationship and leaves a very small bequest to the disinherited person, such as an amount of $10.00. This way, in the event the child wishes to contest the will, the bequest is evidence that you did not merely forget them.
As to why you are doing so, there are two schools of thought on this. One view is that you should specifically state your resons for disinheritance so that your wish to disinherit is clear; the other point of view is that under no circumstances should your will include references at to why you wish to disinherit the person. The thought is that sometimes there are very emotional reasons for doing this, but putting this information in your will, which becomes a public document, can lead to lawsuits against your estate for testamentary libel.
In this case especially, you really should consult with an attorney in your area who specializes in estate planning.