How does one update a Will and add changes?

You can update an exisitng Will by using what is known as a "codicil".  This is a document executed after a Will is in effect.  Although a codicil can be useful in some circumstances, such as when only a small change to a recent Will; it is sometimes advisable to simply begin with new Will to avoid the confusion that complicated codicils can cause.  The general rule is that codicils are useful for any small, inconsequential changes to an exisitng Will.  For example, a name change of one of the beneficiaries, replacing the person named as executor, or additional instructions for your burial. 

Note:  A codicil requires specific phrasing and must follow the same execution (signing) rules as a Will under applicable state law.  

If the changes you are making are substantial or directly affect the original beneficiaries, you should execute a new Will entirley (i.e. not use a codicil).  For example, if the update has to do with a new marriage marriage, a divorce, birth or adoption of a new child, a large change in assets, or including new/excluding old beneficiaries.    

Note:  Whenever you create a new Will, you must revoke all of the old Wills and codicils.  If you do not, the court will acknowledge both Wills and only follow the new Will where the 2 documents are inconsistent (i.e. disagree).    

Frankly, since 10 years have elapsed, drafting a new Will versus adding a codicil might well be in order.  A substantial change in life circumstances and/or finances would most probably dictate this.  You may also wish to consult with a probate attorney to help you write either write your codicil or a new Will so that your wishes regarding the distribution of your estate are respected.