There are currently two options to sync or transfer Minecraft worlds between platforms and devices:

Manually transfer your world. Manually locate your world save on the source device and then copy/paste it into the minecraftWorlds folder of the target device.

To copy your world form the source device:

1. Connect the source iOS device to a computer
2. Download and open iFunbox (Windows, Mac) or iExplorer (Mac)
3. Navigate to the world saves folder: apps/minecraftPE/games/com.mojang/minecraftWorlds
4. Locate your world save folder, and drag or copy it to your desktop
5. Disconnect your iOS device

To transfer/pase the world to the target device:

1. Connect the target iOS device
2. Navigate to the world saves folder: apps/minecraftPE/games/com.mojang/minecraftWorlds
3. Drag or paste the world save folder you copied in step 4 above to minecraftWorlds
4. Disconnect your iOS device

Realms. Subscribe to Realms, upload your existing world, then download it on your other device.

To create a Realm:

1. Navigate to the Worlds
2. Select Create New → New Realm → Create New World

To upload a world to Realms:

1. Locate your Realm in the Worlds tab
2. Select Realm Settings (pencil icon) → Game → Replace World
3. Select any of your existing worlds to upload

Download a world from Realms:

1. Locate your Realm in the Worlds tab
2. Select Realm Settings (pencil icon) → Game → Download World