偷天换日 to steal the heavens and change the day
Explanation
比喻暗中改变事物的真相，以达到蒙混欺骗的目的。
It's a metaphor used to describe secretly changing the truth about something to achieve deception.
Origin Story
话说古代有个贪官，名叫王大人，他贪污受贿，鱼肉百姓，民怨沸腾。为了掩盖罪行，他便想了个办法，偷天换日，把账本上的数字改小，把贪污的钱财转移到其他账户上。他暗中指使心腹，将所有账目改写，把大量的银两转移到了海外的秘密账户。他还伪造了大量的发票和收据，企图以此来蒙蔽朝廷的官员和百姓的眼睛。可是，天网恢恢，疏而不漏，他的罪行最终还是东窗事发，被揭露了出来，等待他的将是严厉的惩罚。
Once upon a time, there was a corrupt official named Mr. Wang in ancient China. He was greedy and corrupt, oppressing the people, causing widespread resentment. To cover up his crimes, he devised a plan to secretly change the truth by altering the numbers in the account books and transferring the embezzled funds to other accounts. He secretly instructed his trusted aides to rewrite all the accounts, moving huge sums of money to secret overseas accounts. He also forged numerous invoices and receipts, attempting to deceive court officials and the public. But in the end, his crimes were uncovered, and he faced severe punishment.
Usage
常用来形容暗中欺骗，掩盖真相的行为。
It's often used to describe the act of secretly deceiving and concealing the truth.
Examples
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魔术师的表演真是偷天换日，令人叹为观止。
móshūshī de biǎoyǎn zhēnshi tōutiānhuànrì, lìng rén tànwéiguānzhǐ
The magician's performance was truly a stunning feat of illusion.
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他竟然能偷天换日地把证据销毁，实在令人费解。
tā jìngrán néng tōutiānhuànrì de bǎ zhèngjù xiāomiè, shízài lìng rén fèijiě
It's really puzzling how he managed to secretly destroy the evidence, changing the truth of the situation in a deceptive manner.