You can change your will at any time as long as you are of sound mind. Major life events such as marriage, divorce, death of a family member or arrival of a new baby are good reasons to consider changing our will.
You may revoke your old will by destroying it, or by writing a new will. If you only want to make minor changes, you may create "codicil," a document that is attached to your will. The same legal formalities are required to create a codicil as a will, and therefore it is wise to consult an attorney about the changes you would like to make.
In the meantime, do not write on your old will, because you may end up invalidating the entire document.