How do I recover deleted messages on WhatsApp?

It's pretty tough to recover them if they're already deleted from the app. WhatsApp doesn't have a "trash" like email where stuff stays around longer - when you delete, it's usually gone right away. BUT there are a few options that might work depending on the situation.

Your best shot is if you have automatic backup turned on in iCloud. If you do, you can uninstall WhatsApp and reinstall it, and the first time it'll offer to restore from backup. Thing is, you'll go back to whenever that last backup was, so you lose anything more recent than that. If the backup's from today, for example, you might be able to get back messages from yesterday that you deleted. Go to iPhone Settings > [your name] > iCloud > Manage iCloud Storage and look for "WhatsApp" to see when the last backup was made.

Another somewhat desperate option: if you were using WhatsApp Web on your computer and left it open, maybe you can find a screenshot of the conversation somewhere else. But if you don't have any of that, then yeah, it really gets tricky. WhatsApp doesn't store deleted messages on their servers for recovery later - everything just stays on your phone.