Demotion in step

I’m a nurse with a VAMC. Over a year ago, I was promoted to a nurse 2, step7. The HR department contacted me most recently and said that my step is incorrect. They are dropping me several steps and reducing my pay by 11k. Granted, this promotion was signed off on by HR and was placed in my eopf. I’m new to this bargaining and litigation stuff, but I would like to know what my next step should be to challenge this since this was a mistake on their behalf. Actually, other employees are facing this as well. Thanks for any helpful input and advice.