Then select model from the EDIT pulldown menu. ( EDIT > MODEL )
In the form that comes up select EDIT INSTANCE (TEXT)
A window comes up with all the spice model parameters. You can change any parameter. Perhaps you want to see how your circuit runs with a high or low beta (BF).
The modified model is saved. SCHEMATICS creates a new library containing the new model. The library is a file called "circuit.lib" in the directory containing the circuit. The model name is comething like q2n3904-X. If you edit the model parameter a second time, another model with a different name is added to the library.
Using EDIT > MODEL you can also select the model you want to use. PSPICE will search all included libraries for the model you select.
This way the nom.lib is untouched. You own the modified models in the circuit.lib.