Thank you for responses, but maybe I didn't explain it properly first time.
Mesh Height adds the height BELOW the reference line. If it has 0 value, the height of the whole mesh is determined by the elevation of the mesh point(s).
I made this Mesh from the isohypses from the topography. For example if I give one line of points an elevation of 15 and the other 20, I will have the shape above the reference line even if Mesh Height is set to 0.
All I want to do is to reduce this elevations by, for example, 5 meters. So 15 becomes 10 and 20 becomes 15.
P.S. My name's also David.