You certainly can! There are a few different options for you – from adding a codicil (an additional document which allows you to make small amendments) to reviewing your entire will.
If you get in touch with your solicitor or the individual who drew up your will then they can advise you of the best way forward for you.
To change your will and leave a gift for Action Foundation, all you need to take is our charity name (Action Foundation) and number (1132051).