A will does not become binding until the person who makes it dies. Until that point the person making the will is not bound at all and can change their mind as often as they like by making a new will.
For guidance on what makes a valid will, see How do I make sure my Will is legal?
As always, we’ll be on hand to give you guidance and checklists every step of the way, so if you have any questions, you only have to ask.