1. There should be a gap under your PC where to put your fingers in (top left image).
2. Put your fingers into that gap and pull (top right image). It may take quite a lot of muscle power to pull the front I/O off. It's easier to do when case stands upright but for better view, i set my case on it's side.
3. After pulling, the front I/O pops loose (bottom left image).
4. Don't pull too far since there are cables going to the front I/O. While this method seems rude, it's the most common way to remove case's front I/O panel, so you can mount/clean front fans (bottom right image).