Android:
- Save the data on your account.
- Uninstall GD.
- Download and install the apk of the texture pack.
- Open it and log in to your account and load your data.
You
must delete the original GD before installing the texture pack because if you don't do it, the texture pack can't be installed because of the changed recourses, GD refuses to "update".
Steam:
- Download the texture pack.
- Open it. It contains a folder called "Recourses"
- Rightclick your GeometryDash.exe and click "Open file path".
- In the opened window, you see a folder with the recourses. Delete it and replace it with the "Recourses" folder of the texture pack.
Doing this on Steam won't reset your data so you don't need to save your data.
iOS:
- Not possible. Even if it was, there's no iOS version of this pack.
I hope I could help you.