Get Rid of Old Messages
Deleting old messages in the Messages app will also help you get some storage back.
To delete whole conversations, open the Messages app and select the three-button menu in the top right corner. Choose Select Messages from the pop-up menu and then start selecting as many conversations as you want.
When you’re done, tap Delete in the bottom right corner and then tap Delete again to confirm.
While deleting text-only messages won’t save you much space, clearing out photos and videos that you’ve sent and received can make a really big difference. You can also do this from individual conversations by opening a conversation, selecting the person’s image at the top and then scrolling down to see documents, photos, and links that you’ve shared with that person. Choose See All beside the Photos section, then Select in the top-right corner; select any photos or videos you’d like to remove and tap Delete in the bottom-right corner.
If you want to go one step further, you can tell your iPhone to only keep your messages for 30 days, so it automatically cleans itself.
All you need to do is go to Settings > Messages > Keep Messages and then select 30 Days or 1 Year. Keep in mind that this will get rid of all messages, so you need to be completely sure you want to do that.