How to Free up space on my iPhone 6s?

How to check the storage on your iPhone and iPad - Apple Support

How can I free up space

1. Delete non-essential apps
2. Remove or optimize excess iPhone photos
3. Remove or optimize excess iPhone videos
4. Delete downloaded/synced music
5. Clear Safari's cache
6. Delete old messages

This would definitely help you --> storage space - Apple Community

How can I free up space on iPhone 6s

The answer has been given a couple times already.