You should review your will roughly every five years and after any major change in your life such as a new child/grandchild or if you move house.
If you are making a minor amendment to your will, you can add a formal amendment, known as a codicil. This must be signed and witnessed in the same way as the will, although the witnesses don’t have to be the same as the original ones. Never write on your original will as it will invalidate it.
If anything substantial needs to be changed, you should make a new will and cancel your old one.