The LM2596 is a widely used component, and its library should be available through various electronics design communities or forums like Electronics-Lounge, or the official Proteus forums. Always follow safe practices when downloading software or libraries from third-party sites.
For exact LM2596, check (search: "lm2596 proteus") or create custom component using above steps.
C:\Program Files (x86)\Labcenter Electronics\Proteus 8 Professional\Data\LIBRARY Transfer Files : Copy and paste the downloaded files into this folder. Administrative Rights
To test if your newly installed model is working perfectly, set up a basic buck converter test bench:
: Observe how the circuit handles different current draws before soldering.