1)
In what way did they rebel against the word of Hashem?
1.
Seforno: Although initially they led Moshe to believe that sending the spies was a strategy to see which was the best way to enter Eretz Cana'an, they now revealed their true intention - to see whether they were capable of conquering the Cana'anim, in spite of Hashem's promise to give it to them - which explained why, the moment the spies told them that the Cana'anim were strong, they were unwilling to go.
2.
Targum Yonasan: By believing the words of the ten wicked spies - against those of Hashem. 1
2.
Oznayim la'Torah: They rebelled against the command - in Pasuk 21 - "Alei Reish!"
