Answer
Wa `alaykum as-salamu wa rahmatullahi wa barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
A Muslim woman does not have to undo her braids for ritual bathing.
In his response to your question, Sheikh Ahmad Kutty, a senior lecturer and an Islamic scholar at the Islamic Institute of Toronto, Ontario, Canada, states:
There is no need for a woman to unbraid or unlock her hair. She can simply wash the hair as best as she can.
This is what we can learn from the Prophet’s wife Aishah (may Allah be pleased with her) who did not undo her hair while performing the ritual bath. She simply used to pour water over her head. She was told that Abdullah Ibn Umar ordered women to undo their braids when bathing. She said, “How astonishing Ibn Umar is! Shouldn’t he order them to shave their heads?! I used to bathe with the Prophet (peace be upon him) from the same pot and I never did more than pouring three handfuls of water on my head.” (Muslim)
It was narrated that Umm Salamah (may Allah be pleased with her) asked the Prophet saying, “I twine the braids of my hair, should I unbraid them for bathing from ritual impurity?” He said, “No. It is enough for you to pour three handfuls of water on your head then pour water on the whole of your body, thus you get pure.” (Muslim)
In conclusion: a woman does not need to worry about undoing the hair while bathing.
Allah Almighty knows best.