Describe two things Buck does to gradually remove Spitz from the lead position in The Call of the Wild.
I think you're talking about things Buck does to challenge Spitz's authority before they have their fight to the death. Buck becomes lead dog after he kills Spitz.
Buck deliberately interferes when Spitz tries to punish one of the other dogs. One morning Sptiz couldn't find Pike after a heavy snowfall, and he was furious with Pike. Once he found him, Spitz went to punish Pike, and Buck flew in between them. Both Buck and Pike attacked Spitz, but Francois stopped Buck with his whip, and Spitz was able to punish Pike.
For the rest of the trip, Buck interfered with Spitz, but he did it when Francois wasn't around because he remembered the sting of the whip and lash. Buck caused all of the other dogs except Dave and Sol-leks to rebel against Spitz so that the team no longer worked together. There was always fighting among them, and Francois had to spend extra time trying to keep the peace until they got to Dawson.