Q: Impossible to upload new ios 9.1 on iphone 4s (the latest version of Itunes is installed) error code 29

Take a look at these articles:

Resolve iOS update and restore errors in iTunes - Apple Support

Get help with iOS update and restore errors - Apple Support

Info from support article:

Check for hardware issues

Errors: 1, 3, 11, 12, 13, 14, 16, 20, 21, 23, 26, 27, 28, 29, 34, 35, 36, 37, 40, 53, 56, 1002, 1004, 1011, 1012, 1014, 1667, or 1669.

Your device or computer might have a hardware issue that stops the update or restore from completing.

1. Check that your security software and settings aren't blocking your device from communicating with the Apple update server.
2. Then try to restore your iOS device two more times while connected with a cable, computer, and network you know are good.
3. Make sure that your security software and settings are allowing communication between your device and update servers.
4. If you still see the error message when you update or restore, contact Apple support.

If you still see the error message, check for outdated or modified software.