Sometimes an update will appear to not be progressing, or have started at all.
This is an Apple App Store problem; please try restarting your device in the first instance.
If that doesn't work you can try the procedure recommended below. Note: First of all make sure you've recently carried out a Backup & Sync from the SwiftKey 'Account' menu.
- Open your device Settings
- Tap 'General'
- Tap 'iPhone Storage'
- Find 'SwiftKey'. Tap 'Delete App'.
You should now be able to download SwiftKey from the App Store again.