the thing attached to the ceiling is called a ceiling rose. It has more wires in it than you might have expected. Some of them are live even when the light switch is "off" so you must turn off the power at the Consumer Unit before you start and until the job is successfully completed.
There are probably about three cables going into the rose, each with three cores; plus another three in the flex. If you have two-way switching from different ends of the room, as also used in halls and landings, there will be more cables and more cores.
You will need a small and a medium electrician's scredriver, preferably the insulated type marked "VDE" which have a hard plastic cover on the metal screwdriver blade to prevent you touching it. Like this but you don't have to buy a set If you have a neon screwdriver, smash it with a hammer and throw it in the bin as it is untrustworthy. You can buy a digital multimeter for around £12 if you like.
Before disconnecting anything take a photo of the way the wires are connected and preferably mark them with dots from a permanent marking pen so you can put each of them back where they were. It is very common for householders to make a mistake that turns the fitting into a fuse-blowing device.
With luck you will only have to remove the three cores of the flex that runs down to your lamp.
After turning off the power, you can unscrew the round cover by hand, and photograph what you find, then screw the cover back on and post the photo here.
Like everything else, it is ludicrously simple when you know how; and impossible when you don't. But if you decide to use an electrician instead of DIY, ask around for personal recommendations (not advertisements on paid-for websites masquerading as recommendations) and verify that they are qualified members of a self-certification scheme that you can check online.