It seems that many people have the problem of recovering their iTunes password. Just in Apple Support Community, a guy said in the following:
"When I am getting ready to restore iPhone 6s from backup on computer, a dialog pops up suddenly and asks a password to unlock iPhone backup. Now I only know that I have forgotten iTunes backup password. So I guess and try several possible passwords, Apple ID password, default password, iPhone unlock code, and Windows administrator password, but they don’t work. How can I unlock iTunes backup for iPhone 6s on Windows computer after iTunes backup password forgot?"
It is really annoying when you forgot iTunes backup password for iPhone 6s/6s Plus, because you can hardly get anything from your iPhone 6s or 6s Plus, including your music, contacts, text message, videos, and etc. unless your unlock the encrypted backup file.
Then what to do? Use Tenorshare iPhone Backup Unlocker! That’s the best way we can recommend to you. Through using this software, you can choose either recover or reset your iTunes backup password.
Step 1.Select/Import iTunes Backup Key Container File to Decrypt. After download and installation, run Tenorshare iPhone Backup Unlocker on your PC. Click "Add" and then double click on the file you lost password from the drop-down box.
Step 2. Tenorshare iPhone Backup Unlocker provides 3 types of attacks, namely Brute-force Attack, Brute-force with Mask Attack and Dictionary Attack. Select the most appropriate one to effectively retrieve iTunes backup file password.
Step 3. When all these are settled, click "Start". iPhone Backup Unlocker will automatically find the lost password to your iPhone 6s/6s Plus backup.
Step 1. Select Encrypted iPhone backup file.Click "Add" and the program will detect and find directory of the backup file automatically.Then click "Enter" button next to the file you want to remove password.
Step 2. Enter the password you just find for your encrypted backup file and click Decrypt to remove the password from your iPhone backup. Your unlocked backup file will be stored under: \My Documents\iTunesRecovery.
After all that, you can get into your iTunes backup again, and get your treasured information.