Buying back my retirement

Can someone explain to me the process of buying back my retirement? For reference i have been in the military for almost 11 years (not quite 11 years since i will be getting out 4 days before my actual 11 year mark. Joined March 19,2014 last day will be march 15 2025) i plan to become a federal employee shortly after getting out. Currently, have the high 3 as my retirement plan. TIA!