Q: password of itune backup

Do you know the old password?

If you have the old password you should be able to connect the device and click the 'Change password' button.

If you don't know the password there is nothing you can do except abandon the old data & erase the device & make a new backup after setting up the device again. iTunes requires the password to change or restore a device. You can make an iCloud backup but that keeps less data than an encrypted iTunes backup.

Apple have some info on encrypted backups About encrypted backups in iTunes - Apple Support

Here you find the information about encrypted backups in iTunes.

About encrypted backups in iTunes - Apple Support

Please refer to the information we linked to…

About encrypted backups in iTunes - Apple Support

Turn off iTunes backup encryption

To turn off iTunes backup encryption, enter the password and uncheck the Encrypted backup box in iTunes. Your encryption password is always required to turn off backup encryption.

You can't use an encrypted backup if you don't have the password. If you forgot your password, the only way to turn off backup encryption on your device is to erase your device and set up as new. Erasing removes all data from your device. If you don't want to erase your device, restore from an iCloud backup instead.

You can't just remove the backup password unless you erase the device. Apple suggest you use an iCloud backup to copy your data & then restore it. I don't know if that will reset the iTunes backup password.

The only consolation is that you do get an unlimited number of guesses for an encrypted iTunes password.