Running 4.1.2 XXELLA , How to custom rom?

Got an old S3 I9300 stuck on XXELLA .
How do i update to compatible modem and bootloadder to install Bliss Pop>?
http://forum.xda-developers.com/gala...bliss-t3010596

Phone is on FULL stock ROM. NO root no cwm.

Phone info IMAGE:
http://i.imgur.com/a6foC4Q.jpg