This is the info from Ceed owners manual for clock setup.
Push MODE button for over 2 seconds the display enters clock setup mode.
The order of setup is Hour -> Minutes.
To go to next setting in setup mode the MODE button should be pressed for under 1 second.
The segments in setting should blink and the value can be changed by up or down button.
Hope this helps you.
It must depend upon model year after posting my original reply I checked on my car the clock needed a tweak anyway. It was the SET button and this did the date and time one part at a time using the adjacent up and down arrows that's for MY2010.
On my 08 C'eed the button you need is labelled "trip" but you cannot use it in calendar mode, use it when displaying mpg or how long till garage etc. Hold for 2 secs as davy says. If it goes to calendar mode to change month / year come out of it. press once to change general display then it should go to clock.