In reply to saurabhchauhan:
Two problems arise over the fact that your my_config class is not derived from uvm_component. Both problem occur regardless of whether you try to use the commend line or config_db::set from your test.
The first is the path used to set the config value. Since it is not part of the component hierarchy, these config items exist at the top level, and are not part of m_env. The second is that the field macros apply settings only for fields in classes derived from uvm_component. You need to explicitly apply the settings.