Actually apply all previous SetPixel and SetPixels changes.
If updateMipmaps is true, the mipmap levels are recalculated as well, using the base level as a source. Usually you want to use true in all cases except when you've modified the mip levels yourself using SetPixels.
This is a potentially expensive operation, so you'll want to change as many pixels as possible between Apply calls.
See Also: SetPixel, SetPixels functions.
var c : Cubemap;
c.SetPixel(CubemapFace.PositiveX, 0, 0, Color.red);
// Do more changes to the faces...
c.Apply(); // Apply the stuff done to the Cubemap.