Does anyone know what type of screw this is?

It's either allen or torx, can't see it clear enough to see detail.

Can't see it well. But I think the middle one is a torx 

i can not see the screw 

How about taking a flash picture so we can SEE it?  Too much to ask?

They are normally used so you can tighten them when you only have access from the side (often when levelling) so you need to find something to slot into the holes on the side of the head and turn it anticlockwise. But close up I can see the main hole is six sided, so an Allen key should do the trick - or a hex in a mixed set of screwdriver bits.

They are not hex-headed screws. They are some sort of special head which in fact I have never seen before but require the right special tool to turn them. Unless you can put your hands on one of those, your only way out is to drill a centre hole and use a screw extractor on them.

Those look like they need an allen/hex key to unscrew.  Most people have some lying around in their junk drawer as they come free with most flatpack furniture.  Why don't you put the call out on whichever social media you use and see if someone will lend/give you one?

A special screwdriver is need to unscrew those fittings. It beats me why they have used weird screws on a desk. As the tool will not be readily available I would use small vice grip pliers to unscrew them and replace them with standard screws.