How Do I carry out an Upgrade? I want to upgrade from Android 4.1 to 4.3 on my Sony Xperia SP smartphone.

Please, how do I go about it to have a successful outcome without losing what I have already on the phone?
There are a number of ways to upgrade your android device.  If your carrier has provided you with an update, you can go under Settings>About Phone>Software Updates.  This will see if there is an update available.  You can then choose to download the update and it will upgrade your device.  I have not heard anything about updates from carriers wiping data.

Method 2 is the unofficial but also beneficial method.  If you are rooted and have an unlocked bootloader you can download stock android 4.3 (or custom ROMs also running on 4.3) and 'flash' the software to your device.  You will need to back up your data before doing this as most ROM developers recommend doing a factory reset when installing.  You can use a number of apps to back up data, but my personal favorite is Titanium Backup.  If you choose to 'flash' a custom ROM, make sure you make a nandroid backup from your recovery before flashing!*