Locating files/service/config.json.template in ODK Central to setup mail server

1. What is the problem? Be very detailed.
I need to config mail server. according to docs i need to change files/service/config.json.template file with my mail server credentials. But i am not able to find the way to do it.How and from where to access config.json.template file ?

2. What app or server are you using and on what device and operating system? Include version numbers.
ODK Central

During setup did you complete this step?

Now you'll need to download the software. In the server window, type git clone https://github.com/getodk/central and press Enter. It should think for some time and download many things. Then type cd central to start working with the software.

You should connect to your server and cd central to get back into the correct folder.

A similar method you used to edit other settings should work for editing the mail server configuration:

Next, you need to update some settings. Type nano .env and press Enter. This will launch a text editing application.

So instead of
nano .env
you should be able to type
nano files/service/config.json.template
and then follow the instructions for Using a Custom Mail Server.