It depends on whether you want to make small changes or completely change your original Will. If you want to change the contents of your Will completely, you will probably have to write a new one and discard the original Will by physically destroying it. If you want to make new changes, but your original Will has already been signed and witnessed, you will have to make an official alteration called a codicil.
A codicil is a document that allows you to amend your existing Will instead of rewriting a new version. Like a Will, it needs to be signed and witnessed in the same way. You do not need to use the same witnesses as in your previous Will. However, if you use a person who benefits from a gift in the codicil or their husband/wife or civil partner who might benefit, then your gift to them will be invalid. You can make minor changes to your Will or alter different sections by using a codicil, but if you are thinking of making large changes, then you should consider writing a new Will.