How did K kill Luv?

of course she could, replicants are not robots, they are "more human than human"

She absolutely could die by drowning. It's made abundantly clear in both Blade Runner and Blade Runner 2049 that replicants are not electro-mechanical robots with human form, but artificially created organic replicas of human beings. Biological not mechanical constructs. They are stronger, faster, tougher, etc. than natural humans, but they are still organic, biological life forms, and they need to eat, sleep, drink, breathe and do all the other things that all biological life has to do to survive.

The scene showing K kill Luv makes it unmistakably clear he drowns her: he holds her head underwater until she can no longer hold her breath, she gulps in a lungful of water instead of air, and that's it. Glug, glug. Lights out. There's no question about it.

I had forgotten that the two movies strayed away from Phillip K Dick's source novel of replicants being androids.