Thanks for getting back to us with that information, katja4. It's appreciated. Since you couldn't locate Instagram in the storage of your iPhone, I'd like for you reinstall the operating system of your device.
How to back up your iPhone, iPad, and iPod touch -- Please be sure to have a fresh backup to keep your personal information safe. In order to back up your Health data, the backup needs to be encrypted. iCloud does this automatically, but iTunes doesn't. If you'd like to use iTunes to back up your iPhone, refer to this article: About encrypted backups in iTunes. Also, if you don't use iCloud Photos, be sure also import the photos and videos on your iPhone to your computer: Transfer photos and videos from your iPhone, iPad, or iPod touch.
If you can't update or restore your iPhone, iPad, or iPod touch -- When an application isn't working as expected, a good step is deleting and reinstalling the app. Since you can't delete the Instagram on your iPhone, I'd like for you to reinstall iOS. The section titled "Put your device in recovery mode and set it up again" will walk you through all the steps to reinstall iOS software on your device. See if Instagram has been removed.
Let me know how that goes, and take care!
