“A Tzaddik needs to do Teshuva?! – That’s ludicrous! Why would a Tzaddik need to do Teshuva if he never ever does or even thinks of anything wrong?”
Well, now that we know that Teshuva means to return to Hashem, not (only) to fix problems, we can understand this.
Our Neshama becomes distant from Hashem on two levels:
Just coming down to the world, without doing anything, already makes the Neshama “far” by stopping to feel that Hashem is the only reality. This, however, is caused by Hashem Himself for a reason.
Already in the world, the body and the Nefesh Habehamis (the “animal soul”) through their choices, can make matters worse and cause the Neshama to become even more distant, losing the closeness that can be achieved even in this world, through the fulfillment of mitzvos.
Corresponding to these two levels, Teshuva also has two levels:
Restoring the Nashama to “mint condition,” to the state that it was given to us in. Our deeds are corrected and we no longer make it more distant than it already is.
Bringing it even closer to Hashem by creating a deeper connection and improving its standing beyond the way it came here.
[Obviously the second level of Teshuva is something that can only be done if the first, “lower” level has been completed.]
This is the kind of Teshuva Tzaddikim do.