JellyJelly, 6 bytes (non-competing)bytes
OBU¬ḄỌ Explanation:
OBU¬ḄỌ Main link: z O Convert z to a list of character codes. B Convert the codes to bit lists. U Reverse the bits (not the lists). ¬ Invert the bits. Ḅ Convert back to decimal. Ọ Convert back to string.