Scanwatch horizon - battery life very poor (and getting worse)

I have my Scanwatch Horizon since 20th June 2023.
During first months I charged the watch every Sunday while I do my bikeride for convenience (my bike computer was monitoring heartbeat etc during that time). Back then, when I started charging I always had remaining battery above 20% or more, so I would have done 9-10 days without charging.
For the past few months the battery life has plummetted so much that it does not last one calendar week aymore. I need to charge every 5 days or so.
I use exactly the same functions and configuration from day 1.
I understand this is a failure of the battery and it should be elegible for a battery replacement under warranty. How should I proceed to claim this replacement?