How does Squealer excuse changing the Commandments in Animal Farm?
As a propagandist, Squealer's job is to convince the other animals that any decisions made by Napoleon are correct and that to disobey is to risk reverting to human control. When the pigs move into the farmhouse, Squealer makes the first substantial change in the Commandments: he adds "with sheets" to the injunction against sleeping in a bed.
"A bed merely means a place to sleep in. A pile of straw in a stall is a bed, properly regarded. The rule was against sheets, which are a human invention... You would not rob us of our repose, would you, comrades? You would not have us too tired to carry out our duties? Surely none of you wishes to see Jones back?"
(Orwell, Animal Farm, msxnet.org)
Squealer uses two techniques here: first, he outright lies and pretends that the addition was always there. By making the alternative seem seditious, he convinces the others that the Commandments were not changed, but that they just "don't remember" the addition. Second, he argues semantics, acting as if the original Commandment -- which is really about animals adopting human traits -- specified sheets instead of the entire bed; by moving the goalposts, Squealer is able to bypass the issue of whether the original Commandment still applies.
In the ending paragraphs of chapter III, the ruling pigs begin to use propaganda in order to justify their violation of the seven commandments; the pigs violate the commandment of equality when they take all the milk and apples for themselves. The other animals are angry, but Squealer is sent to pacify them. Squealer uses lies in order to deceive the other animals into believing that the pigs take all the milk and apples because they want to serve the other animals and to protect the farm from Jones.