Inventor can't back-calculate the result when you have constrained a part to all three axes and then decide one is backwards. Even though there IS a solution, Inventor can't find it. What you can do is suppress one of the other two, correct the one you want to reverse, and then un-suppress the second one.
Later as you learn, you will discover the uses for suppressing constraints so this is as good a time as any to go looking for that (in the feature tree, right-click on the constraint listed under the part in question).
To have more control over the position of the part when it is constrained, I avoid axis constraints, exactly because I can't control the orientation. When constraining to planes, I can select "mate" or "flush" which allows orientation relative to the positive normal direction of the plane. Not so with axes. They don't always have an obvious positive direction.
STF