Well, it is whete you have installed it, obviously. When you are using ubiboot you can put your OS'es wherever you want, but usually mmcblk0p4 is used for Nemo/Sailfish.
(for example I have both Nemo and Sailfish, so my Sailfish lives on mmcblk0p5...)

As for how to replace the Sailfish installation with a new one, just mount your Sailfish partition somewhere, delete the content and extract the new FS image there, no need to do anything complicated...