How to set default display name ??

Hello again..
I came to know from the internet that showing username as display name is a risk for the website. When i had not any community and i was the only member, i changed the display name and nick name. But when i have a community, i want to change the display name as first name only or the both first and last name and nick name as provided by the user during registration by default. So that if the user does not change it, username is not showing. Is it possible with wpforo??
Or i have to do that with any other plugin. By the way, i am also using ultimate member plugin.
There is another related query. For the similar reason, can i change the mentioning name with the display name??

The page I need help with: [log in to see the link]