How did Camp Green Lake change Stanley?

Stanley Yelnats from Holes was a rough, directionless kid prior to Camp Greenlake. His time there shapes him, though not through the conscious actions of the leaders there. Stanley grew to be a hard-working individual who would persevere and finish working after his time. Additionally, he stood up for what he believed in and learned to care more about others, particularly because of his relationship with Zero. His time there shapes him and helps him mature into a much more caring, dedicated individual who helps others, works hard, and stands up for his beliefs, when, beforehand, he was aimless, solitary, and uncaring. So, while it was unintentional, Camp Greenlake was a successful rehabilitation facility and program for Stanley and some of the others.

In the Louis Sachar's novel Holes, Stanley Yelnats is sentenced to spend time in Camp Green Lake because he is convicted of stealing a pair of sneakers. Stanley did not actually steal the sneakers, he just happened to be in the wrong place at the wrong time. Stanley and his family are convinced that bad things happen to them because of the curse placed on their ancestor a century ago. When Stanley arrives at Camp Green Lake, he is complacent and passive, resigned to accept whatever the fate has in store for him.
However, after spending some time digging holes and interacting with his fellow "campers," Stanley begins to see that things happen as consequences of his actions. It is his choices that make the boys respect him, and he makes a friend, Hector Zeroni ("Zero"). Stanley teaches Zero how to read, and Zero helps Stanley with his holes. This is not taken well by everyone else, and as a result Zero runs away to into the desert.
Stanley worries about Zero, as it is impossible to survive in the desert for long without food and water. Eventually, he takes his fate into his own hands and goes looking for him, ignoring the danger involved. Both Stanley and Zero survive, come back to camp, and locate the treasure that the camp's Warden was trying to find by making the boys dig holes. By the time Stanley's parents arrive to get him out of camp, Stanley has the confidence to refuse to leave without Zero. When Stanley leaves camp, he is no longer complacent; he knows that he can shape his own fate.