Note 5 running Lolipop upgraded to Marshmallow?

So I repair phones screens and can do most of the basic things with cell phones.
But one thing I see a lot of people doing are flashing different Android OS's onto their phones.
Which I believe is called ROMing?
So my biggest question is, can I flash a Note 5 from Lolipop to Marshmallow and not affect my cell phone service?