In Animal Farm, tell how the animals were forced to accept Napoleon's changes.Once Napoleon gains absolute dictatorial control, he changes laws to...

You have three questions here, so I will answer your first one:  Tell how the animals were forced to accept these changes.

Napoleon changes the rules at will, as you have noted.  He does so for his own benefit; for example, the pigs move into the farmhouse and sleep in beds, and they take more than their fair share of milk.  Whenever the other animals think about protesting these changes, Squealer comes around to deliver the daily message of propaganda from the pigs.  He reminds the other animals that the pigs need more benefits because they do more of the work (well, the thinking work anyway; they certainly don't do any of the manual labor).  He also reminds the animals that if the pigs weren't in charge, the farmer would return, and they would all be miserable.  Usually, this reminder serves to quiet the protests from the other animals.  However, if the need arises, Napoleon can always bring out his dogs, to provide the muscle behind the message.  With these large dogs growling in their menacing way in support of Napoleon, none of the other animals dare to protest further.