I used many POS systems while I was waiting tables and going to college. That was over 15 years ago. This looks like one of the earlier ones I used. If I were to attach a date to the one you've listed, I would say it came from 1992.
So, what makes it look dated? There are no graphics, the font looks old, and it looks like the entire UI was computer generated.
In the 1990s the toolset for creating rich media was limited. I don't know what programming language your team is using, but if you're getting ready for a product makeover, you may want to see what you can do to convince your team to switch to a newer language that makes working with rich media a cinch. The programming tools your team uses will have a huge impact on what you can and can't design for your system.
Where I'm currently living, POS systems have taken a giant leap forward in the last three years. A POS company called Clover has wiped out the competition in Mountain View, CA where I work. Why are they doing so well? The design of their system uses all of the modern advances in technology. https://www.clover.com/
If I had to convince my engineering team why we needed to change, I would send them to Clover. If they can't understand why Clover is good design, I would look for a new job.