Rav Wolbe writes (Alei Shur vol. II pg. 442) that we find three different aspects of teshuva described by the Targum: Firstly, "Return, Yisroel to Hashem your G-d" (Hoshea 14, 2) is translated by the Targum, "Return Yisroel to the fear of your G-d". Additionally, "Take words with you and return to Hashem" (ibid. 14, 3) is translated as, "Bring with you words of confession and return to the service of Hashem." Finally, we find the pasuk "Wash yourselves, purify yourselves" (Yeshaya 1, 16) explained by the Targum to mean, "Return to the Torah."
Rav Wolbe elaborates on the Targum's explanations. The literal meaning of the word "teshuva" is to return, for the entire concept of Teshuva is returning and becoming closer to Hashem. How does one accomplish such a feat? How can one bring himself closer to his Creator? This can be accomplished in any one of three ways; by returning to the Torah, the service, or the fear of Hashem.
Torah is not only the elixir of life; it is also the most foolproof way of combating the Yetzer Hara. As Chazal tell us, Hashem declares, "I created the Yetzer Hara and I created the Torah as an antidote." If one contemplates the impetus for his aveiros, he will find that each and every one of them is rooted in a laxity that he has taken toward some area of Torah study. We might be able to trace them to the fact that he hasn't set aside time for Torah study, or that he settles for a superficial understanding when he could delve deeper. Maybe he hasn't studied the applicable halachos or he hasn't reviewed what he has studied. It is possible that he should be spending time studying mussar or even the siddur (from which one can glean emunah and bitachon) and he hasn't taken the time to do so. There is no better day to do "teshuva to Torah" than Yom Kippur, the day we received the Torah in the form of the second set of luchos.
The teshuva mentioned in conjunction with avodas Hashem, is achieved by the proper attitude toward performing mitzvos. Most mitzvos are aimed at purifying, sanctifying and elevating our bodies from the physical to the spiritual. The constant performance of the mitzvos, day after day, leaves small, but nevertheless, indelible impressions upon our bodies. However, there is one catch. The performance of mitzvos only leaves these impressions, when one pays attention to what he is doing. Avodas Hashem performed out of rote, even if done for many years, will leave a person on the same spiritual plane that he started. Doing teshuva by performing mitzvos with a sense of purpose is another way of returning to Hashem.
"Returning to the fear of Hashem," means returning to the study of the fear of Hashem. Out of all the various aspects of Torah study, the Yetzer Hara expends special effort to prevent people from studying about the fear of Hashem (i.e. mussar). He knows just how powerful this study is, and he wages his war accordingly. One's teshuva must include time allotted toward the study of mussar.
There is no time more conducive to making a chesbone hanefesh (spiritual reckoning) and doing teshuva than the Aseres Yimei Teshuvah in general and Yom Kippur specifically. All three forms of teshuva mentioned above are viable ways of coming closer to Hashem. We now have the knowledge of what teshuva is. Let us take this knowledge and implement it in these ways in order to return to Hashem and achieve a teshuva shleimah!
Gmar Chasima Tova!