Is Recovery Mode necessary to flash to newer Android version manually?
Note: Installing the custom recovery using the tools below requires a rooted Xperia Go. Generally root isn't always necessary and a device can be rooted by flashing a pre-rooted ROM for example, and a recovery flashed via fastboot as another example. Also note it can brick your device. I take no responsibility for the below if your device turns into a shiny paper weight

The below is for a Windows Machine only:

To install a custom recovery, clockworkmod-recovery in this case: 


Place the archive you just downloaded to your Desktop.
Extract files from the archive to your Desktop.
Turn ON the USB debugging function on your device by going to Menu&gt;&gt;Settings&gt;&gt;Developer options&gt;&gt;USB debugging.
Connect the device to your PC using the USB cable.
Double-click on the install.bat file and the utility should launch.
Follow the instructions displayed on your screen.
Once Recovery’s installed, reboot the device.
You’re all set!