How do I set the default view mode to 'flat' in newbb and how to change that for existing users

The first question is what is your newbb version?
You can find your newbb version easily in admin area by putting your mouse over the newbb icon.

For newbb1:
go to system --> preferences --> general setting and change "Default Comment Display Mode" to "flat"

For newbb2:
go to newbb admin --> preferences and change "View Mode of the Threads" to "flat"

For changing the view mode for existing users to flat run this query in your database via phpmyadmin:

if you using another perfix change "xoops" with your xoops_perfix

This Q&A was found on XOOPS Web Application System : https://xoops.org/modules/smartfaq/faq.php?faqid=355