Not sure on your model but usualy ther is one of three ways. The head unit should have a button marked clock somewhere on it, have a look it may be very in very small writing and may be off to one side of the button.
1/ Try holding this button down with the radio on for about 5-10 seconds the display should change and possibly flash use the volumn controls to change time.
2/ Try holding this button down as you turn the head unit on and/or off.
3/ Try holding the button down as you turn the ignition on.
Sorry been a while since I have had to do this as most repairs I do I avoid removing the battery and where I have to I use a special battery in the cigarette lighter.
I'm sure someone here will know exactly what to do.
Kind regards Guy