Hello Srujan and welcome to Advanced Installer forums,
Unfortunately, I am afraid that this can not be achieved using our predefined support for a
"Text File Update" operation.
However, this can be achieved through a
custom action. For instance, you can have a script which modifies the content of your already existing config file based on the data from your ComboBox control.
This script should be scheduled in after the
"Add resources" action group, as during this action group the files are copied on the target machine.
Hope this helps.
Best regards,
Catalin