Use your middle mouse button to rotate the view so that you can see the height of the selected mesh.
Choose a vertex is at the height you want the mesh to be flattened to, then, holding the Shift button on the keyboard, use your right mouse button to de-select then re-select it.
This will change the Active Element to the vertex, which is now highlighted in white with the Axes centered on it.
Press S (for scale), then Z (to constrain the movement to the Z axis), then 0 (for the height relative to the Active Element), then Enter (to finish the scale).
This will scale the height of all selected vertices to that of the one which was selected.
Your contributions are licensed under the open source Creative Commons license.
Insert a new line below
Insert a new line above
Indent current line
Unindent current line
Delete current line
Navigate to next step.
Navigate to previous step.