IOS – I forgot a password for iPhone 5C, error 4013 if try to restore

The password used to encrypt the backup file might have been saved to the user's keychain.

1. from /Application/Utilities open the Keychain Access utility
2. make sure that on the left side "All items" is selected under categories
3. in the upper right search field enter "iphone" and then look for a keychain item named "iPhone Backup" of kind "application password". If there is no such item, you´re mostly out of luck
4. double click on the "iPhone Backup" entry and in the new window that will open, click on the "show password" checkbox
5. you will now be prompted to enter your password (the one your user has on your Mac) to show the iPhone backup password; click Allow (once). You should now see the iPhone Backup password in clear text.

I don't think windows has an equivalent store for saving the password. If not, you will have to try brute forcing it in iTunes - just me methodical - write down all sorts of passwords that may have been used - brainstorm a bit. Then try them one by one. Hopefully with the pressure off a bit you will stumble upon the pass phrase that was used.

If an app was installed by someone else (say, a friend or family member) using their Apple ID, you will be required to type in that Apple ID's password to reauthorize the app to your device now that it has been wiped & restored.

If it is a free app it is asking you to authorize, you can also remove the app and re-download through the App Store using your own Apple ID account.