Ash-Shaykh Saalih al-Fawzaan, may Allah preserve him, said :

حلق الشعر وتقليم الأظافر لا يجوزان في عشر ذي الحجة لمن أراد أن يضحي، أو أن يأخذ شيئًا من شعره أو أظافره في عشر ذي الحجة، حتى يذبح أضحيته؛ لنهيه صلى الله عليه وسلم عن ذلك ‏؛ كما في حديث أسماء رضي الله عنها، أما مجرد تسريح الشعر الذي لا يحصل معه زوال شيء من الشعر؛ فلا بأس به‏. 

“Cutting the hair and trimming the nails is not allowed for the one who intends to make a sacrifice  during the ten days of Dhul Hijjah. (Similarly), or to remove any piece of hair from himself or from his nails during the ten days. (This is) until he slaughters his sacrifice due to the prohibition of the Prophet sallahu alayhi wa salam from that, as comes in the hadeeth of Asmaa, may Allah be pleased with her. As for hairstyling only which doesn’t cause pieces of the hair to fall off, then that is okay”

Reference : Muntaqaa min Fataawa al-Fawzaan.