Just to clarify, this is the credentials used to connect to SMTP. You only need to change this if Discourse is having problems sending emails or you want to change email providers. This isn’t your user email account.
Assuming you still need to change it, you can edit this line in app.yml:
DISCOURSE_SMTP_USER_NAME: test@*******.***
Then rebuild Discourse:
$ sudo ./launcher rebuild app
Alternatively, you can re-run discourse-setup and enter the new username when prompted.