For the ghusl of janabah a woman does not need to unravel her plaits; it is enough that she pours water over her head three times so it reaches the roots of the hair, then washes the rest of her body. Umm Salamah told the Prophet that she braided her hair tightly and asked whether she must undo it for the ghusl of janabah, and he replied that it was sufficient to pour three handfuls of water over her head. Most scholars extend this ease to the ghusl after menstruation as well, though some recommend loosening the braids for the ḥayḍ ghusl as a precaution; in every case the requirement is simply that the water reaches the scalp.
Informational, not a personal fatwa. Consult a qualified scholar for rulings on your situation.