How do I restore instant messages from iCloud?

Unfortunately, if your messages were synced with iCloud, it will not be possible to recover them. When you delete something on your phone that is synced with iCloud, it disappears completely.

If your messages were not synced with iCloud but were located locally on your iPhone, then you can try to restore from an iCloud Backup from the time they were available on the phone.

You can read here how to restore from an iCloud Backup:

Restore your iPhone, iPad, or iPod touch from a backup - Apple Support