Not a barometer at all then !
There may be adjustment screws on the back but the cheap thermometer and hygrometer I have cannot be adjusted although the matching barometer can.
Nothing in the instructions ?
Its innards look the same as this one which is apparently adjustable https://www.marinestore.co.uk/Matt_Chrome_Channel_Thermometer_&_Hygrometer_.html
Also looks like the Plastimo one