Change a variable to the opposite?

[Megahertz]

That is an "exclusive OR" (XOR) with a byte of 00001111. So when you input 00001001, it will return 00000110.

Here is the truth table on page 79 of the manual.

A    B   A ^ B
0    0      0
0    1      1
1    0      1
1    1      0

If only A or only B is 1, then result is 1.
The ^ operator is commonly used to invert selected bits:

This would my my answer as well.