Up the tension.
I don't think cables "stretch" in any meaningful way. I believe that it is just the cables seating in their clamps.
Go to the rear deralleur, and turn the nut one-quarter turn COUNTER-clockwise. This will retract the housing from the deralleur, causing it to take up more cable and consequenty upping the tension. You may need to turn it more than one-quarter turn, adjust and take it for a test ride.
Also, have you cleaned the drive train. After cleaning the chain, cassette, chainrings, and deralleurs, the shifting on my bike became much better. My Sora bike shifts incredibly smoothly with a clean drivetrain...