The contrapositive of a conditional "if p, then q" is "if not q, then not p." Example: Take the conditional, "If it's raining, then it's pouring." The contrapositive is "if it's not pouring, then it's not raining." In classical propositional logic, whose conditional is known as the material conditional, a conditional and its contrapositive are equivalent, i.e. either both are true or both are false.
In Bob Seger's "Against the Wind," one finds the line: "Wish I didn't know now what I didn't know then." With contrapositives in mind, we can show that Seger also wishes he knew then what he knows now, and that these wishes are equivalent!*
For Seger not to know now what he didn't know at that beatific, past time he has in mind, the following would have to be the case**: if Seger did not know x at that time, then Seger does not know x now.
The contrapositive of this conditional is just: if Seger knows x now, then he knew x at that time. That is, Seger knew then what he knows now. So, Seger's claim that he wishes he didn't know now what he didn't know then is simply a more poetic way of claiming that he wishes he knew then what he knows now. (At least, according to classical logic...)
* I assume for this discussion that if someone wishes p, and q is logically equivalent to p, then that person wishes q, and define the wishes to be thereby equivalent.
** Two small points. (1) Technically, we are drifting into predicate logic here, and the variable x is universally bound, i.e. the italicized sentence should begin "For all x..." (2) I interpret Seger's statement in a logically precise way, which might not match ordinary usage. For example, if someone said "Bill ate what Jim ate," one natural way (perhaps the most natural way) of understanding this is that Bill and Jim ate the exact same things (Bill ate it if and only if Jim ate it), not just that if Jim ate it, then Bill ate it. My claim that the relevant wishes are equivalent is not affected by this point, as long as we understand the wishes in the same way, and don't "switch" between them.