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If you're keeping it in the same position, not very difficult. Take the old light off and you'll likely find a ceiling rose termination underneath. It should be screwed to a joist to take weight of light.
You can then attach new light to this ceiling rose termination, either by screwing cover straight on (lightweight light fitting) or placing a bar behind to take weight of heavier light fitting.
If you want to move light to new location on ceiling, then it's involves finding the closest joist, routing cable, installing new ceiling rose and patching up old hole which can be difficult to blend in with ceiling.
How difficult is it to change a ceiling light fitting? Or is that a how long is a piece of string question?