IMO, 6.x is not necessarily a good thing....
I had it for a short time on my Note 4 before it died....
My daughter's S5 is working just fine like it is, and she don't want it messed with.
The wife's S5 is still on 4.4.4 and has not seen the Internet since I gave it to her two years ago....
and she is happy with it the way it is... for her, it is a telephone, period.