To resolve the issue of accessing your information from the My Book of Clients Salon app after upgrading to your iPhone 15, follow these detailed steps:

1. Check for iCloud Backup:
- Go to Settings on your iPhone.
- Tap on your name at the top to access your Apple ID.
- Select iCloud, then tap on Manage Storage.
- See if you have a backup that includes the My Book of Clients data.

2. Search for Alternative Access:
- Even if the app isn't in the App Store, check if there is any way to access your information through their website or a secondary application that might have replaced it.
- Search online for the official website of My Book of Clients Salon and see if they provide any web access to your data.

3. Contact Support:
- Locate the contact information of My Book of Clients support. They may have solutions for accessing your information if the app is no longer available.
- Look for support via email, phone, or a contact form on their website.

4. Use Device Data Sync:
- If you had your iPhone set to backup your data to iTunes or Finder on your computer, connect your iPhone to the computer and open iTunes (or Finder if you're using macOS Catalina or later).
- Select your device and choose the option to restore from backup, which may allow you to recover the app data and information saved prior to your upgrade.

5. Explore Alternative Apps:
- If restoring the app or content isn’t working and customer support does not resolve your queries, you may need to look for alternative apps that offer similar features. Research thoroughly to find an app that could migrate your data or is compatible with your needs.

6. Email Recovery Attempts:
- If you have backup emails related to My Book of Clients Salon, check those email accounts for any instructions or links that could help you retrieve your information.
- Look through all associated folders including Spam, Promotions, and any other relevant folders for missed messages.

Following these steps should help you regain access to your information from My Book of Clients Salon or find an alternative solution. If you encounter specific issues during these processes, providing those details will allow for tailored advice.