Check whether it applies to you at all before looking at remedies. Where it stands, the classical response is Mars remedies — Hanuman worship on Tuesdays above all — and, traditionally, Kumbh Vivah, the symbolic marriage that transfers the dosha.
Manglik Dosha is Mars in one of the houses that classically strains a marriage. It is the most feared and most over-diagnosed condition in Indian astrology — a large share of charts carry it in some form, and a large share of those have one of the standard cancellations.
The mantra, the weekday fast and the charity are safe for anyone — they address Mars without amplifying it. Red Coral is not: a planetary gem strengthens whatever the graha is already doing, so it needs a chart reading first.
Tuesday. Fasting, charity and mantra japa for Mars all belong to that day.