Once your Will has been completed, you cannot make changes by simply altering it. However if you only wish to make a few simple changes this can be done by way of a codicil.
If the changes you wish to make are fundamental, then it is best to make a brand new Will to avoid any misunderstanding between the wording of your original Will and the words you might substitute by codicil.
It is vital that you take responsibility for reviewing your Will on a regular basis to take into account changes in the law and taxation, and also changes in your personal status. For example if you marry or divorce, this will have the effect of revoking all or part of your Will.