Please try the following CSS code. Put it in Dashboard > Forums > Settings > Style > "custom CSS code" textarea.  

@media screen and (max-width: 620px){
  #wpforo #wpforo-wrap #wpforo-menu .wpf-res-menu {
    display: none;
  }
  #wpforo #wpforo-wrap #wpforo-menu .wpf-menu li a {
    padding: 5px 11px;
  }
  ul#wpf-menu {
    display: flex !important;
  }
}

Please check on incognito mode, because mobile browsers have a hard cache you cannot see the changes immanently.

how do I change from drop-down menu to desktop menu set up?

I only want to have

1. forum

2. login(user name if the user is already loged in), 

3. message

I really do not like the dropdown menu style on the mobile menu.

for the mobile menu, log in is almost hiding. some of my users are old people. I will have to make it easy.

Would you let me know how to change it? 

Thanks