How do I?

I'll try to explain it the best I can so here I go,

Remove the carb cap and slide like the second pic here: http://www.blasterforum.com/do-yourself-20/aftermarket-carburetor-cap-idle-screw-install-22174/

Then remove the throttle cable from the carb slide, and tip the slide upside down and the needle will fall out

Carefully remove the c clip on the needle and reinstall the c clip on the position you want and put the needle back in the slide

Last install the throttle cable on the slide and put the slide and cap back on