Level of repair for rotting outdoor stairs
Obviously you have to replace the bad treads. The bigger issue is the stringers. Unless you remove all the treads and trace one it'll be difficult to accurately match the current shape. For that reason, and assuming that the rot doesn't substantially weaken them, your best bet is patch blocks. 

I'd cut lengths of 2x6 to about 16" and screw them to the stringers, level with each tread cut and flush with the riser cut. They can extend out the rear somewhat, saving fitment effort. Use four to six 3" corrosion-resistant screws on each, through the stringer first. This puts the primary grab in the new wood. Do this on one side of each stringer as needed.