At what point can I change the locks?

[Raymond Felipe]

One tenant is supposed to be moving out today. My other tenant said she hasn’t been home since yesterday but her stuff is still here. If she isn’t back by next week, can I change locks and hold onto her belongings until she asks for them? 
I also already told her that all belongings left behind will be thrown out and she said ok sure

Also should I send her a message again to remind her it’s move out day? I sent her one last week