The Rebbe's guidance on fasting and self-affliction as part of teshuvah, and the chassidic path he taught instead:
In our generation, not through fasting. Teshuvah today is achieved not by fasting but by adding in Torah study, prayer with the congregation, tzedakah before davening and other mitzvos (IK 4151b). Spiritual growth comes through learning — especially Chassidus — rather than self-affliction (IK 2780, 8680). Chabad tradition discourages frequent fasting and all-night vigils (IK 2583), and voluntary fasts should be replaced with positive alternatives (IK 4287).
Health comes first. Keeping the body healthy is itself serving G-d and a Torah obligation; fasting that harms one's health is not the way (IK 1151, 1673, 1683, 2462, 6244). Skipping meals weakens prayer and study (IK 3399). Serving G-d with joy and staying healthy take precedence over voluntary fasting (IK 1897, 3769, 5489). Real refinement comes from inner discipline — guarding one's speech, sight and hearing — not from mortifying the body (IK 1673, 4196, 4614, 4720).
Tzedakah instead. Halachic sources set a sum of tzedakah to give in place of a fast of teshuvah (IK 199). Taking part in a Pidyon HaBen can, according to some sources, count in place of many fasts (IK 3046). Where fasting is part of teshuvah, it should follow halachic guidance rather than personal stringency, with tzedakah and daily learning (IK 6003); partial fasts together with extra tzedakah may be advised (IK 8525). A "fast of speech" has its place but does not replace the traditional fasts (IK 5836).
Balance and joy. Teshuvah should be approached with balance — neither too lenient nor too harsh — and guidance should suit the individual rather than follow rigid rules (IK 2337, 2503). Sadness over sin is answered with the contemplation taught in Tanya (IK 3576). Teshuvah itself should be done with joy (IK 1226, 6513), focusing on bringing G-d pleasure through positive deeds rather than on one's failings (IK 6510, 2948).
See also: Teshuvah – How to do teshuvah; Health – Fasting and afflicting oneself.