The word invert is used mostly for the act of turning a thing upside down. You can also use it for an act when you are arranging something in the opposite order. The following sentence can make you understand the term well as it is used in the context: you just have to place a plate over the tin containing cake and invert it. The abovementioned sentence says that the person has to put the plate on the tin of the cake and then to take the cake in the plate, has to put it upside down.
Another example that can make the meaning crystal clear is following: In questions you often find that the subject and the verb are inverted. The sentence says that verb and the subject usually interchange their positions when you convert them from affirmative to interrogative.
Another example that can make the meaning crystal clear is following: In questions you often find that the subject and the verb are inverted. The sentence says that verb and the subject usually interchange their positions when you convert them from affirmative to interrogative.