how to downgrade ios 7 to 6.1.3

Not possible.

Apple does not support iOS downgrades.

Apple does not support downgrading the iOS.

You would need to ask Apple that.

We are users just like yourself.

Do you have a specific problem with iOS 7 that we can help with?

Wi-Fi settings grayed out or dim:

1.Verify that airplane mode is off by tapping Settings > Airplane Mode.

2.Reset the network settings by tapping Settings > General > Reset > Reset Network Settings. 
Note: This will reset all network settings, including Bluetooth pairing records, Wi-Fi passwords, VPN, and APN settings

3.Ensure that your device is using the latest software. To do so, connect your device to your computer and check for updates in iTunes.

4.If your issue is still unresolved, perform a software restore in iTunes.

5.If restoring as a new device does not resolve the problem then you will need to make a Genius Bar appointment or contact Apple for repair.

So, turn it off.

Just as it says, you need to disable Find My iPhone:
Settings > iCloud > Toggle Find My iPhone off

After you turn off the Find My Phone, I suggest you read the manual.

Go to Settings, iCloud, Find My Phone - turn off. Enter the password.

On your phone tap the Settings app (Gears icon)

then iCloud:

Then turn off Find My iPhone