Is there any way to replace the battery in the Fenix 6x pro sapphire?

It's not replaceable by the consumer, you'll have to contact Garmin about it. I'm sure they'll figure something out for you, even being outside of the warranty period it's very unlikely they'll just tell you "sorry, you're hosed"

SM said:

Garmin hasn't effectively even acknowledge that there is an issue, much less fix it in their infrequent updates.

the5krunner who I think posts here has commented a few times on battery problems, and DCRainmaker mentioned random and rapid battery drain in his Forerunner 745 after 8 months article recently - he thinks it's software rather than hardware.

Anyway, if it's software with two prominent reviewers now acknowledging rapid drain issues, we may see a bit more focus on it. Certainly one of the most frequent types of posts we see here (and also other places) - in my view Garmin are going the wrong way with this - asking users to do resets is going to lose the possibility of doing a root cause analysis of the drain cause, especially if a reset does actually resolve it.

I had this problem with my 6x pro too.  Bottom line is Garmin offered to sell me a refurbished 6x pro for USD170 to replace my watch after resetting it failed to resolve the battery problem.  The refurbished replacement would come with a 3mth warranty.  While my watch is out of warranty, I agree with you.  It doesn't really make sense to purchase something this expensive knowing that it cannot in practice be repaired after the one year warranty expires.  I also don't think paying for a refurbished watch with a questionable history/future is a solution.  The solution in my mind is for them to be made repairable.  I'd be happy to pay for that if it's out of warranty. I have owned many Garmin watches over the years, but this was a bit of a wake up call.  

Just chiming in that I’m having the same issue. Fenix 6X less than two years old and battery life is at about 50% what it once was. A full charge now lasts only a week, even with no GPS use. Getting someone else’s used refurb is not a solution. Sell us the batteries or offer a battery replacement service. 

There is a great fenix 6x teardown webpage and the replacement batteries are cheap ($12).  If/when I need to do this it looks really straightforward to swap the battery, so that's what I'll do.  A little bit of silicon grease on the gasket and it should still be fully waterproof.

It is not replaceable as per Garmin, hence not possible to buy new OEM battery.
But replacing it is not that difficult - https://www.youtube.com/watch?v=La73m_t0Sr0

There are two options of batteries that you can find. Slightly used original, or not original, search for it by part number - 361-00126-00
These you can find for around 20 € 

Done! Battery ordered. I'll let you guys know how it goes. 

Where from did you buy it, could you share a link?

The place that punkgeek suggested above: interlightus I had previously ordered the same one from typebattery.com but it was being shipped from Hong Kong and could take weeks to arrive, so I cancelled that order. YMMV.