Basically he he made the decision that apartheid needed to be reformed, he dismantled petty apartheid laws and then announced the end of apartheid altogether. He released leaders of the black resistance, including Nelson Mandela. In addition, he signed the Pretoria Minote.
Desmond Tutu used peaceful protest and his position in the church to fight against the apartheid system. He made speeches speaking out against it, and helped those who fell victim to the ways of apartheid.