Refresh WSDL

I want my SOAP clieny to automatically detect the wsdl and update the definition?? Is it possible?

If your SOAP Client is configured with a WSDL URL then the SOAP Client's endpoint and Form Input configuration will automatically reflect URL and request schema as defined in the WSDL. If the endpoint or request schema from the WSDL were to change, then the SOAP Client will attempt to update itself automatically. In particular, if the schema for the request payload were to change, the Form Input configuration adjusts itself automatically, attempting to preserve existing values in the request payload using heuristics. At runtime, the WSDL is read automatically with Form Input values adjusted automatically prior to sending the request. From the UI, the WSDL is read when a SOAP Client editor is first opened or a refresh can be forced using the Refresh WSDL button.

Updating SOAP Clients to reflect latest WSDL definition can also be managed using a feature in SOAtest called Change Advisor. For detail, please see Change Management.

I will tell you my problem. I have a .txt file from where my WSDL definition is being read in Parasoft(I have given the file path in WSDL tab). I have written a logic to update the .txt file before I execute a row.

If you have multiple WSDL documents then you must keep them as separate files and not a single file that you update or replace. Then you would make your SOAP Clients use a variable for the WSDL URL. The values for your WSDL URL variable can then be declared in different "environments" in your tst test suite. For more detail and explanation for how this works, please see the following:
Configuring Testing in Different Environments
Running Tests in Different Environments.