A beneficiary can rewrite the provisions of a will (insofar as those provisions apply to him or her) by means of a document known as a ‘deed of variation’ (sometimes referred to as a ‘deed of family arrangement’).
If there are several beneficiaries and they want to rewrite more or less the entire will, they all have to be party to the deed of variation.
If there is a trust in the will, it may prove impossible to rewrite the will, or at least the section of the will containing the trust, using a deed of variation – but other options may be available. 
Sam has died, having made a will which leaves his entire estate to his son Jed. Jed is independently wealthy and wants to pass the estate to his brother ...