Teshuvah means repentance. Maimonides (Rambam) recorded in the Mishneh a formula for complete repentance.
What is repentance? The sinner shall cease sinning, and remove sin from his thoughts, and wholeheartedly conclude not to revert back to it, even as it is said: “Let the wicked forsake his way” (Is. 55.7); so, too, shall he be remorseful on what was past, even as it is said: “Surely after that I was turned, I repented” (Jer. 31. 19). In addition thereto he should take to witness Him Who knoweth all secrets that forever he will not turn to repeat that sin again, according to what it is said: “Say unto Him.… neither will we call any more the work of our hands our gods” (Hos. 14.3–4). It is, moreover, essential that his confession shall be by spoken words of his lips, and all that which he concluded in his heart shall be formed in speech.
Maimonides, Mishneh Torah, Chapter 2 (c.1176 – c.1178 CE)