Using any other data cable might cause connection problems.
Now open Flashtool>Drivers>Flashtool-drivers.exe and install theFlashtool, Fastboot & Xperia M drivers from the list.After installingSony Flashtool, open the Flashtool folder from the drive where you installed it.You must have installed Sony Flashtool.Check your device’s model number in Settings > About device. Do not try this on any other phone, that may result in bricking your device. As mentioned, this firmware is only for Xperia M C1904/C1905.All you need is, you need to follow this guide being very careful.
In the following post we are going to update Sony Xperia M C1904/C1905 to latest stock Android 4.3 Jelly Bean 15.4.A.1.9 firmware. Flashing a firmware manually can also become handy when you’re stuck with your firmware and you have no other way to get rid of the problem except for flashing the stock firmware once again. Just in case you haven’t received via OTA or via Sony PC Companion yet, you may consider updating your Xperia M to this latest firmware manually using Sony Flashtool. As usual, these updates are always region specific.