Turn the bit off or on and update the shift registers. The bits are numbered from right to left so the rightmost bit is 0 and the leftmost is 15.
To turn a bit off:
uint16_t mask = 1 << bitNumber;
leds &= ~mask;
The '~' operator inverts all of the bits in 'mask'. The '&=' operator turns off any bit in 'leds' where the matching bit in 'mask' was a one (before being inverted to zero).
To turn on a bit:
uint16_t mask = 1 << bitNumber;
leds |= mask;
The '|=' operator turns on any bit in 'leds' where the matching bit in 'mask' is a one.