Hi I am having the usual square down light installed in my false ceiling.

Recently one of the down light is not working anymore and does not light up even after changing the bulbs. Therefore I suspected it the down light fixture that is spoil.

How do I replace it? Or should I call an electrician since it is installed in the false ceiling?

Since it is your first time, pay & get an electrician.

I assume yours is not LED type (since you mention replacing the bulb). Probably the ballast is 'kaput'.

The electrician should be able to remove the light, check & replace for you.

Watch how he do it & next time you can DIY.

I suggest since it's your first time, get a professional electrician to trace the problem for you. It could be the downlight or the driver issue. Since it's a downlight, I supposed when you take down your downlight(downlight with very strong clips), your false ceiling will get damaged. So getting a professional electrician, they should be able to patch up your damaged false ceiling as well.

very good advice indeed