Hi
Usually ceiling spot lights are held in place to the ceilings upper surface by two sprung arms. These are held against the light fitting as its pushed up through the hole and spring out downwards and support the light fitting when its finally in place.  
Applying an even downward force on the lights rim should slowly allow the fitting to be eased out and the sprung arms forced upwards allowing the fitting to be completely removed.
Turn power off at mains then use something solid but thin to go between rim of light and ceiling carefully ease the fitting downwards. A couple of dinner table knives or a metal wallpaper scraper or if you have them a pair of 12 inch steel rules are ideal as they are thin but strong, but all can be used a levers.

And if you can manage to get one out without pulling away some of the plaster than you're a better man than me.

And if you can find one identical to replace it....you're very very lucky.

Some makes can make a mess if not carefully done.  JCC for one.  The heat from halogen lamps causes the can of the light to become hot, which then makes the plasterboard brittle.  The heat also damages the lamp holders.  They are a horrible design.  If you do pop any plaster around the hole, the lip of the fitting is too small to cover it.  Such a crap, rushed design when fire rated fittings first came out.

I would highly recommend removing them and fitting some Collingwood H2 Pro 700T, or 550T.  The 700 is equivalent to a 50watt halogen, and just 8watt.  The 550 closer to a 35watt.  They are excellent fittings.