Jelly, 6 bytes

OBU¬ḄỌ 

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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.