You Can Find out How to Apply it Right Now via this Easy Tutorial .
4- Open the XPERIA Flashtool, click on flash and choose downloaded firmware, than click ok. Important - if you do not want to delete any data of your phone, just uncheck flashing userdata.sin file. All data will remain in your phone, but sometimes it can cause additional troubles. So it is better (and I recommend that) to do the backup of your personal data, than proceed with clean instalation and then restore your personal data.