An existing will cannot be altered but a gift can be added either by a codicil (a short supplementary will) or by making a new will. Suggested wording for a codicil (which must be signed and witnessed in the same way as a will):
I (full name) of (full address) declare this to be a first codicil to my will dated and made the day of (date, month, year). I give _______ to Christian Compassion Ministries UK of 34 Woodland Rise, Welwyn Garden City AL8 7LF, Registered Charity No 1084104, for its general charitable purposes. I further direct that the receipt of the Chairman of the Board or other duly authorised officer shall be full and sufficient discharge to my executors.