As was stated, the executor cannot change the will...period BUT
there are often issues where the will ambiguous enough the executor has the authority to make decisions regarding assets.
An example;
Bob dies. His will is simple. Joe manny and sue each receive 1/3 of his net estate.
Since there are no specific assets listed to be given to any particular legatee/devisee, the executor can generally divvy up the assets as s/he chooses as long as each legatee/devisee receives enough assets to be worth 1/3 of the total net estate.