What is it called when you say things backwards?

Hint: Saying backwards indicates that we are reversing the sentence in such a way that the meaning of the sentence remains the same.

Complete answer:
We can say things backwards in many ways. The most common ones are as follows:
1) Inversion:
Inversion refers to the reverse of the syntactically right order of subjects, verbs, and objects in a sentence as a literary device. This form of inversion is also known as anastrophe, which comes from the Greek word anastrophe, which means "to turn around." In English, sentences are formed in a reasonably rigid order, usually subject-verb-object.
For example: Inversion of the sentence: “Yesterday I saw a boat” is “Yesterday a boat I saw”.

2) Chiasmus:
Chiasmus is a grammatical and logical arrangement between two or more clauses in which the grammar and definitions are inverted. Inverted parallelism is shown by chiasmus, a figure of speech.
For example: Chiasmus of the sentence:” She told me she isn’t coming back” is “She’s not returning, she said.”

3) Antimetabole:
Antimetabole is a figure of speech in which the first half of a sentence's words or clauses are reversed and echoed in the second half of the sentence. Antimetabole is a Greek term that means "against metabolism."
For example: Oh he will, will he?

Note: A palindrome is a word or expression that is the same forward and backwards, whereas a semordnilap is a word that is read backwards and becomes a new word. For example: “civic” is a palindrome word. “god” is a semordnilap word. Reverse of “god” is “dog” and hence the meaning is changed.