To edit voxels, Voxel Edit Mode needs to be selected.

Desktop mode:

Click left mouse button to add voxel. Click middle mouse button to remove voxel. Hold mouse button and move mouse to add/remove voxels in a single plane. Plane is determined by the direction where you are looking when first voxel is added/removed (for example look downwards for horizontal plane.

VR mode:

Pressing trigger adds a voxel. Pressing trigger while holding grip removes it. Hold trigger to add multiple voxels in a single plane. Initial direction of controller ray determies the plane. Hold both grips and move your hands together or apart to change size of edit sphere.