They don't. Unsigned integers are unsigned, by definition, and therefore always positive.
If you try then what will happen is that the bit pattern of -9 will be stored in the integer and interpreted as a positive number. Negative numbers are stored in two's complement, which is like 1's complement (where you flip all the bits) but then you add 1.
11110110=1's complement of 9
11110111=2's complement of 9
So if this is put into unsigned 8-bit storage, then the result will be interpreted as 247.