For Stock Rom you need rooted phone.
There are many apps, you can use e.g. "Root Explorer" from Play Store.
I think there is a free version available.
Procedure for Root Explorer:
1. copy wished .apk to Storage on phone.
2. start Root Explorer
3. go to .apk you want to copy in system/app
4. select it and copy
5. go to system/app
6. in upper bar in the middle you can see mount status. (-ro)
7. change it to -rw by clicking on it and select item -rw
8. Superuser app flushes: grant access
9. Paste .apk in system/app
10. Restart your phone and have fun.
Feel free to ask if further problems occurs.
AND PLEASE DO NOT OPEN NEW THREADS FOR QUESTIONS YOU HAVE ALREADY ASKED IN EXISTING ONES.