SetLineTexture
From ZDoom Wiki
void SetLineTexture(int lineid, int line_side, int sidedef_texture, str texturename);
Usage
SetLineTexture will change the specified texture on all lines with the line id number specified (assigned by Line_SetIdentification). For readability, fields 2 and 3 (line_side and sidedef_texture) have some definitions in zdef.acs:
SIDE_FRONT - front of the linedef (front sidedef)
SIDE_BACK - back of the linedef (back sidedef)
TEXTURE_TOP - upper texture of sidedef
TEXTURE_MIDDLE - middle texture of sidedef
TEXTURE_BOTTOM - lower texture of sidedef
Examples
script 1 (int line1, int line2, int line3)
{
SetLineTexture(line1, SIDE_FRONT, TEXTURE_MIDDLE, "-"); //remove middle
SetLineTexture(line1, SIDE_FRONT, TEXTURE_MIDDLE, "-"); //floating texture
SetLineTexture(line2, SIDE_BACK, TEXTURE_TOP, "BFALL1");
SetLineTexture(line3, SIDE_BACK, TEXTURE_BOTTOM, "FOOTEX01");
}

