For example, you could do something like this:
1. Figure out what your users are using the software for. What are their tasks and workflows? See if you can arrange to observe how users currently interact with the system.
2. Figure out what your software actually does. Make note of anything that seems to be clunky or difficult. Research current UIs, workflows, and get inspired.
3. Figure out where the easiest improvements lie. Would it make life easier and better for your users if you exposed a common operation that is currently buried? Is configuration or personalization cumbersome?
4. Mock up the changes you'd like to make. Some folks use wireframing tools, some use Adobe Illustrator. Pick a tool and run with it.
5. Let the folks in charge know that there's an easy adjustment that could be done. For example, give them some basic stats on how much time their users spent before doing task A, then show them the potential improvement in the user's time on tasks.
6. Go to 1 until they refuse to make changes that improve the usability of the software. Then, go find something else to do.
And this is just my opinion as someone who works with UX practitioners: I believe that improving organization and adding user-centric workflow optimizations are more important than just slapping some lipstick on a pig. However, if that's all that you put in front of the folks in charge, they may be underwhelmed. UI adjustments are just a small part of improving UX, but their importance shouldn't be discounted as a persuasive tool, as they are the most visible.