wpforo_date($post, 'd/m/Y g:i a'); 
wpforo_date($post, 'd-M-Y g:i a');

I changed the top line to the bottom line.

There is another way to do it. Instruction here:

• Navigate to Dashboard > Forums > Settings > Features admin page, enable the "WordPress Date/Time Format" option
• Navigate to Dashboard > Settings > General admin page, choose the Custom Date Format option and set it d-M- Y
• Use the following CSS code to make the characters of the month uppercase

  #wpforo #wpforo-wrap .wpf-right .wpforo-post-content-top > span {
     text-transform: uppercase;
  }

Please don't forget to delete all caches and press CTRL+F5 before checking.