Question [Q] MMS.apk missing?

I have a stock Nexus 4, not rooted. I'm trying to update the MMS.apk to a version I found online, to customize the app to my liking.
I'm told to use Root Explorer, go to /system/app, and replace MMS.apk.

I went there to try it, and can't even find MMS.apk. My stock messaging app works perfectly fine, so I know it's somewhere...

What am I doing wrong?

I've attached a picture of what things look like.

EDIT: I found a search function on Root Explorer, which tracked down the file in /system/priv-app.
I apologize for making this thread. I did plenty of research before posting here, but never considered the search function. (I don't use Root Explorer normally. I downloaded it just for this.)
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