This is a very easy task if you are familiar with root access and your device is running a custom recovery like TWRP, Philz or CWM.
First you need a full gapps package, this will be ready to be flashed via the recovery and needs to be provided by OGapps.
( The reason it need's to be provided by OGapps is because Open Gapps already has a removal script that will run prior to installing )
The Gapps Installation Log :
- mounting system
- removing old installations
- installing open gapps
- updating system
- unmounting system
- Done !
The above is just a example, but generally what you'll see. It removes the old google apps first.
Ok, so now you've installed Gapps you'll need to manually remove software that didn't get removed.
This is usually application's that you've updated, even though the SYSTEM has a new APK, the DATA still has an updated application installed, so you'll nees to remove any updates to apps applied from the Play Store previously.
I've never actually had to remove an update from DATA myself, but it's recommended in case of problems... however the OGapps package seems to clean everything up before installing.
Android is Linux with Java & XML interfaces.
It's conveniently named Android to identify the people that joined these thing's together.