Hello, I'm very new here and I've tried searching for an answer to my question manually with no luck.

I'm from Bulgaria and I've recently changed my S6 Edge's motherboard (10 days after my warranty expired). The "new" motherboard is from a Telenor (GBL) phone, it isn't locked. The problem is that Telenor doesn't update their firmware-the last update is for Nougat is in August and their loading screen when I turn my phone on and off is very annoying.

My question is:
Can I change my CSC to another bulgarian carrier and if yes, how to do that?

SM-G925F
AP: G925FXXU5EQH7
CP: G925FXXU5EQH7
CSC: G925FGBL5EQI1