This is alfa romeo giulietta, in front of the throttle body.
You just undo it with a suitable size spanner or socket, probably 6, 7 or 8mm. It looks to be one of the few without a screwdriver slot or phillips screw head.
If it is rounded or seized, I have found the most time-effective solution is to "crush" the screw-locking mechanism which releases the strap... Trying to lever up the strap to get cutters etc in the gap usually damages the rubber.
It seen that is rounded, try to use a normal plier, if that does't work use a vise grip plier (not sure if it is call like that) if it is really stuck, spray some penetrating fluid you can use wd-40 but it isn't the best choice, or try to aply some heat, just be carefull with plastic parts and rubber hoses