I want to detect stairs or elevation using ultrasonic sensor, anyone has an idea , to how to do that
I would imagine you want to differentiate stairs from a wall or furniture. Because stairs are largely standardized, they should all be pretty predictable. You could mount the ultrasonic sensor on a servo motor to tilt up and down to determine the height of what is in front of it using a bit of trigonometry and if it is within a certain range, stair = true. There are probably better ways to do this more reliably, i.e. with a webcam, Raspberry Pi and AI.
Too little information. No constraints on the system? From the single sentence, sounds like only need to worry about stairs going up, not down. How far away must detection be? Stair tread rise and run are fairly standard, Generally, 7 3/4" rise and 10"-11" run.
Two ultrasonic sensors, with transducers pointing horizontally might do it, but will depend on range. One at couple inches above floor, to monitor nearness to wall or riser, and second somewhere around 10" off floor might work.
A single sensor on a servo might work, given a small deflection sweep angle range, but it will need to be close to stairs and positioned at the rise height of the stairs. The reason is you have to consider where will the reflections from the transmitting transducer end up? You also probably want the transmitting transducer on the bottom, and the receiving transducer on the top in this case.
Carpeted stairs likely will not return an echo.
If you want to detect stairs going up or down, put the sensor out in front and facing down. If the distance to the 'floor' suddenly increases, you are at the top of some stairs (or a cliff). If the distance to the 'floor' suddenly decreases, there is something like a stair in front of you.