Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
Applying water to every part of the body during ritual bath is compulsory except inside the eyes. Hence, it is an obligation to apply the water to the hair in order to wash it. If the water reaches the scalp then undoing the braids is not compulsory. If the braids prevent water from reaching the scalp then undoing them is compulsory as mentioned clearly in Al-Bahr al-Raiq and other books of Fiqh.
Allah knows best.