The mixture screws are at the lowest point of the carburetors at the leading edge, right in front of the float bowls. They only really affect from idle to about 1/8 throttle at low engine speeds, so if you're too rich at higher engine speeds or larger throttle openings you'll have to do something else.
If you're rich at full throttle, you might consider removing the snorkels from your airbox, or cutting open the airbox. Removing the snorkels takes less than a minute and is easily reversible.
Who did the rejetting on your bike? What were they trying to accomplish? There is a good chance that they just put the wrong jets in and you'll need to open the carburetors back up to fix the problem.