Not sure this question makes sense. There's no "default model". Each instance of a transistor in a spectre netlist will instantiate a model - which is in turn a definition of a particular set of parameters for one of the built-in devices (such as bsim3v3, bsim4, psp, ekv etc). Originally posted in cdnusers.org by adbeckett
Normally design kits will have a set of symbols, each of which will be set up to netlist with a given model name. Those models will be defined in some files which get referenced via include statements into the spectre netlist.
To change the definition of a model, you'd have to create a model file which has model statements in to give an appropriate set of parameters for whichever model you want to use - this is not something an end designer would normally do, since device characterisation is a non-trivial task. You can't just say - OK, I want to change to use EKV now - it's not as simple as that!
Perhaps I've misunderstood the question - if so, please provide more detail so we don't have to guess what you really are asking...