Changes in WebConfig file for Sandbox Instance.

Please follow the snap attached below to make changes in a web.config file. In the highlight line, please change the URL with below mentioned URL.
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1. '/c' for enterprise endpoint, e.g.:

a. https://test.salesforce.com/services/Soap/c/11.0

2. '/u' for partner endpoint, e.g.:

a. https://test.salesforce.com/services/Soap/u/11.0