Where is the lambda sound sensor in a Volvo V40 1997 model my warning light is always on and can it be reset?

A Main Dealer (or good Volvo Independent garage) can read the fault code that has caused the light to come on. They can also clear the codes from the car.
However, if what has caused the light to come on in the first place is not fixed (faulty Lambda sensor, other faulty sensor etc), then the light will be back on again.

You can also get a code reader for your car and read and/or reset the codes yourself, but again some remedial work is required to fix the issue causing the fault.

The sensor is usually placed in the exhaust pipe somewhere. There may be two sensors, one before the Cat and one after.
Without further details of the car (engine size, model year, saloon/hatch) finding further information is difficult.