Can I upgrade RAM on late 2013 retina Mac Book Pro?
No. The RAM is soldered to the logic board in all of these models.  



Here is the logic board from iFixit's Teardown of the low end 15'' model (2.0 GHz, 8 GB RAM, 256 SSD). 

Inside the yellow box are 16 x 512 MB DDR3 SDRAM (8 GB total) memory chips. As you can see, they are soldered directly to the board and cannot be replaced without replacing the whole logicboard.