Can I show users first name only

Yes, you can open the template files in the plugin.
– If you using the Pro version, you can find the file here: /plugins/dw-question-answer-pro/templates/styles/default/
– If you using the Free version, you can find the file here: /plugins/dw-question-answer/templates/
And find the following code: 

get_the_author()

and replace with the following code: 

get_the_author_meta( 'first_name' )

Note: 
To keep your customize when update the plugin, you can create a “dwqa-templates” folder in the theme folder then copy/paste the files in the default folder into the “dwqa-templates” folder to edit.