Here's how to do it on a 206. May be similar for your Citroen (same PSA group).
Press left button for about 2 seconds until display flashes - should be language. Press again and hours on clock will flash. Press right button to increment hours; press and hold to cycle through lots. When hours set, press left button again; minutes will flash. Press right button to increment minutes. When done, leave for about 10 seconds and display will revert to normal and save changes.
ChicksFan, May 2013