How do I find my wifes iphone from my iphone /find my' app.

Hi,

Before the latest iOS update, I could log in as me or log in as my wife to find my/her phone.

Now the new update is in place, the app is now 'find my' and I can't seem to log out as me and log in as my wife in order to find her phone when she loses it around the house - I'm sure the fix is simply, but I can't find it.

Thank you,

Jon