You could do this in a couple of ways, one of which I use and have tested successfully and the other one equally looks promising and effective. Namely these are:

1. SyncMe Wireless
2. Rsync server and client combo (requires an android sync client and PC (server side) client.

I will try to describe how these methods solve your situation

1. Using SyncMe Wireless application

SyncMe is an excellent tool to copy, move, sync and backup your mp3, pictures and documents over Wi-Fi, (USB tethering, cellular VPN or wired network) to your computer or NAS device.

The biggest advantage of this tool is that nothing to be installed on your computer. Additionally you can sync automatically as long the device is connected to the Wi-Fi by creating automated jobs. The only downside perhaps, is that it currently supports SMB1 protocol which has been associated with some vulnerabilites and disabled in recent versions of Windows (but won't be much of a big problem since you use Windows 7)

Prerequisites

• Sharing must be enabled on your computer, the simplest way to do this on Windows is to right click the folder you wish to synchronize (the backup directory) select Share With and go with the instructions.

Instructions

• The app provides a step-step method of setting a scheduled backup and you simply need to follow the on-screen instructions:
• After download and installation, open the application, connect to your wi-fi network and tap on "Add computer" on the bottom of the screen, here it scans the IP range of the router until it detects the PC or you can manually enter computer IP:

• After a successful scan, you'll be able to see your PC name listed and input a user account (make sure its password protected, also turn on password protected sharing in advanced sharing section on your PC.

  Now you need to add a sync/backup job

• Tap on the PC name and select Add sync folder:

• Here you will be presented with the following screen which will enable you to configure the origin and destination folder for the sync/backup job:

• Now select the device (origin) folder in this case internal storage /storage/emulated/0 (or similar) and the computer folder (i.e destination folder)
• Next you need to tweak the Copy to parameters to suit your backup job:

• When done move to Schedule option, here select the automatic job back up parameters that suit you:

Note: The app has four premium (paid) features in this app: 1) Schedule with router 2) Weekly schedule 3) Include/Exclude directories and files, new extensions and regular expressions 4)

That should do it. Remember to run the job initially to "test" if you configured correctly.

Disclaimer: Tested and works 100%, I am not related to developer of application, and not related in any other means.

2. Rsync

Second option is using rsync, which provides a very fast method for bringing remote files into sync.

Firstly take a look at the post prior to gain a better understanding of how this works: Backup Android to NAS with Rsync

Prequisites

1. Essentially here you need a Windows rsync client such as DeltaCopyor Acrosync - Rsync GUI for Windows or any other even native terminal-based rsync.

2. Android rsync client e.g Syncopoli

   • Configure the virtual backup directories on the target PC, i.e setup the server and the folder using your rsync windows client.

   • If its a bit confusing follow the step by step method here or there

   • Setup your android rsync client by choosing origin and destination folders.

Configure Syncopoli In the parameters change :

• server address to your NAS IP
• protocol to SSH
• private key to the absolute path to the private key on your Android device (i.e /storage/emulated/0/data/rsynch.key.db)

In the Action page, create a task :

• Local to remote (for backup on NAS)
• set a profile name
• set the source folder to backup (i.e. /storage/emulated/0)
• set a destination