How do i change the orfus-tube on 1999 pontiac grand prix?

A/c work is not exactly a Do it yourselfer job with out special tools and equipment. You have to recover the freon form the system first, locate the section of A/C line that houses the orifice tube, Seoerate the two tube sections and remove the orifice tube, Coat the rubber bushing on the new orifice tube with a light film of PAG oil and install the same way it came out, Install new o-rings to the seperated lines , put lines back to gether, using a vaccuum pump draw the a/c system down to 25- 30 inches of vacuum,(mercury is the correct term) then recharge a/c system with proper amount of r-134a. This is really a job best left to a repair shop but it can be done by a D I Yer with proper tools and equipment.